Nebula Shaman (+some advice)

SOME ADVICE 

Good in the starting hand in general:
Murloc growfin
Hex
Turbulus
Mixologist
Birdwatching
Flight of the Firehawk
Bumbling Bellhop
Hagatha the Fabled
Triangulate

Some advice about your starting hand against every class and some more strategy advice:

Against Demon Hunter:
All I have to say is: good luck. They’re simply too fast and have all the possible ways to make your defenses useless. I’d try to keep the Hex in the starting hand, cheap cards and the taunts that spawn a copy like the Bumbling Bellhop or the Frosty Décor, help to counter Red Card, but overall it’s not a favourable match up. Zilliax Deluxe 3000 is useful too. If you survive long enough. Mixologist can help you cleaning their board. 

Against Paladin: try to have a low cost starting hand. If you’re against the Quest Paladin, go face as much as possible. Don’t trade if not strictly necessary in the beginning: they’ll trade to defend themselves and to free their board to make more room for bigger murlocs. Ignore the fact they can give them the draw death rattle with Braingill. For me it didn’t make a big difference. I managed to win against them way more going face every chance I got, then trading to play around that card. And one of them actually died from fatigue, drawing that much. Against them Shudderblock it’s almost impossible to use so be careful. If you manage to use it, might be worth it with Turbulus and Bumbling Bellhop. Or with Nebula turned into a Fairy Tale Slime generated by Hagatha the Fabled, or Zilliax Deluxe 3000 if you’re in a good position. With Nightmare Lord Xavius, try to get chargers, or taunts with reborn or big buffs, mainly.
Against Aggro Paladin, goes the same for the starting hand, but here you have to trade as much as possible and make sure to clean their board as much as possible. Here, your taunt minions are crucial. 

Against Warrior: For the starting hand, Hex. Mainly for Testing Dummy and Hamm the Hungry. On Hamm the Hungry may be useful to prevent them from resummoning it with Hydration Station. Hex will obviously likely bring a froggie back when they’ll use it. Blob of tar works great to quickly get rid of Tortolla. Other advice, I’d say try to get lots of armor and try to bait them into using Brawl with the minor stuff. They don’t really expect cards like Merithra for example. Some don’t even expect Nebula, or Nobundo’s Galaxy lents+Nebula. If you want you can consider using the Galaxy Lents with Hex: Hex when used through the Lents, allows you to target and turn into a frog also Elusive minions, so you can target their Tortolla and Zilliax Deluxe 3000 for example.

Against Mage: Try to get Hagatha the Fabled and Elise the Navigator. With Elise, try to get the location that allows you to discover a spell. Lately basically everytime mages have a secret, it’s going to be Counterspell, so it’s a good idea to get a spell to sacrifice from Elise‘s location. Also the rush minions, the one that summons a copy of a minion of yours, and the one that gives +attack to your hero and minions, are useful to help you testing for Counterspell. Which by the way is crucial because it can really screwed you up, especially if it counters Nebula: you lose one whole turn. When you discover the spell through the location, make sure to get a spell that’s cheap enough (the location usually discounts them). Once you’re sure it’s Counterspell, use the discovered spell and then freely cast Nebula. Against mages, try not to use the Coin if you have it. Keep it for Counterspell. Nebula turned into a Fairy Tale Slime generated by Hagatha the Fabled, is also great to manage to cast it without any risks.
Quick guide on Mages secrets in standard and the best ways to test for them with this deck:
Ice barrier: try to have many minions on board, so that it will be easier to have someone able to attack face and check if it’s the Ice Barrier. Also the Blob of Tar is good for this because the 2 minions it spawns may survive more easily if they kill the main minion and be able to attack the turn after. Elise‘s location with the rush minions or the copy of your minion, can help too with this. It’s also useful the option to give your characters +attack because you may be able to test attacking with your hero or totem’s too. Also useful, minions discovered through Nightmare Lord Xavius with charge. 
Oasis ally: your minions with rush, the rush minions from the location and the +attack given to your characters through Elise‘s location can help if you have no surviving minions from the turn after on board to help you test.
Explosive runes: Just play a minion. But be careful: when you want to play the Sleepy Resident and you want also to attack with some minion already on board, make sure to attack first, because otherwise the runes will kill the Sleepy Resident and get your minions to sleep before they get the chance to attack.

If it’s a Protoss Mage, the ideal would be to Triangulate one Frosty Décor and one Nebula. You’ll need lots of armor to survive against the Colossus. They’re also useful if you get the +attack with the Elise the Navigator location. Against them in general, try to buff minions and do a lot of pressure whenever you can as soon as possible. Sometimes this also forces them to play the Colossus when they’re still too weak, just you try to defend themselves against your board. The best minions to get against them with Nebula are the Splitting Spacerock and the Mo’arg Forgefiend.

Imbue Mage: Incindius+Shudderblock are effective to try to clean. With them you have to try to have a good start so even if they’re useful, I’d not suggest to keep them In the starting hand. Splitting Spacerock through Nebula is useful to counter all the damage they can deal with the heropower, and same goes for Mo’arg Forgefiend and Frosty Décor. 

Against Priest: Hex or Triangulate to try to get Hex are great in the starting hand. Hex is useful against the bandage otk priest and the protoss priest, in particular. Also good against the quest priest.
Against them, to counter minions they may have got from the Mothership, Mo’arg Forgefiend is useful to counter Colossus and Splitting Spacerock against Carrier.

Against Hunter: Try to have hex and Blob of Tar in the starting hand in case is the buff one with Mythical Runebear.

Against Death knight: Hex in case of Bonechill Stegodon.
If it’s a normal control dk, with the Elise the Navigator location, try to get the rush minions to get rid of the Bloated Leech. Also the one to give +attack to your characters works well, usually against them. 
Splitting Spacerock works pretty well against Corpse Explosion.

Against Shaman: I don’t find many at all. And only seen a singular Nebula one so far, and very different from this deck (more defensive). Against Asteroids Shamans, try to get lots of armor and try to have always as many minions as possible on the board. Against the quest one, it’s really dangerous only if they get more then 2 adapts on the minions with Ashalon but you can still manage. In that case, Shudderblock+Incindius combo is fundamental. Getting lots of Nebula, same. Fundamental. And with Nebula, I’d try to go for 2 Meadowstrider to resist fatigue in case his amalgams get the elusive adaptation and you can’t Hex them. They’re probably one of the best targets for Hex even in the beginning when they’re still not too threatening, especially if you don’t have an aggressive enough hand to close the match soon, and you think you’ll get to fatigue.

Every other class: for the starting hand I’d say just try to get the ones I listed in “good in the starting hand in general”. For now I don’t have more specific advice, but I’ll update if I will.

Good combos:
Nobundo’s Galaxy lents+Nebula: allows you to cast a 9 mana spell for free, and you could do it twice in a turn, using the original spell and then tapping the Galaxy Lents.
Nobundo’s Galaxy lents+Triangulate: can be useful to multiply Nebula, Hex and Incindius’s Eruption, mainly.
Nobundo’s Galaxy lents+Hex: it hallows you to choose as targets also elusive minions.
Elise’s location+Nobundo/Incendius: if you get the option that summons a copy of one of your minions on the battlefield, used on Nobundo it would allow you to have an extra Galaxy lents location, OR with Incendius, it will ensure ensure an extra buff on the Eruption spells, that would deal a minimum of 3damages.

Shudderblock+Nebula turned into a Slime through Hagatha: to fill the board casting it 3 times. You’d have a board full of elusive big taunts+ a 9-9 minion.
Shudderblock+Incendius: 15 eruptions can always come in handy. This deck doesn’t rely on boardclears, the only actual boardclear is the Ceaseless Expanse. Outside of that, the way you clear, is through trades with your minions and Hex, so a few eruptions do help.
Shudderblock+Zilliax: especially useful when your health is low, but useful in general. You’ll get 4taunts with divine shield and rush, so they’ll likely manage to help you clear and protect you in the next turn too, and be a threat, especially if buffed through Birdwatching, Flight of the Firehawk, Turbulus or Xavius.

Shudderblock+Murloc growfin: 4 8-8 (or more, if buffed) at once, and with rush. Help to clear the board and given their stats, they’re also a threat.

Obviously you can decide to try to use Shudderblock in combo with other minions too, but I think these are the best. I also wouldn’t suggest to use it with Elise because it’s quite slow to create 3 custom locations. Probably you’d barely make it in time.

Ceaseless expanse+Merithra: or any other board filler you have.

Extra advice:

Birdwatching: if possible Try to get Bumbling Bellhop or Murloc growfin. They’re the ones that’s more worth buffing. Also on Zilliax obviously it’s not bad. This obviously, if you don’t need some other card in that moment.

I would suggest discovering only one Splitting Spacerock per match. it’s useful against some strategies, if you have the Elise’s locations that buffs your characters works great and it’s great to make sure you always have something on the board even when they clean, BUT you risk to have a board full of small minions and no room for your big ones, especially for Merithra.

Stay as far away as possible from Octosari.

Try to get enough different 8drops for Merithra. Remember to try to get also taunt ones. Even if Nebula will give them taunt, keep in mind that Merithra will summon them in their vanilla form, so you’ll need some taunts. The best ones are Mo’arg Forgefiend, Meadowstrider, Al’Akir. Tar Tyrant ad Primordial Drake not bad but quite defensive.
If you played the Murloc growfin and Zilliax, you’ll need to try to get 4-5 different minions from Nebula.

Cards you could consider too:
Parrot Sanctuary, a second copy of Mixologist, a second copy of Sleepy Resident, a second copy of Birdwatching, Observer of Mysteries, Dreamplanner Zephrys, Dirty Rat, Endbringer Umbra.

This is all I have for now, I hope you found it useful! If you have any other question feel free to ask, and consider to vote if you like the deck!