There’s been a change of heart by Blizzard in terms of how your ranking will be displayed on your friends list. Before, they were only going to show your Standard format ranking, which was kind of a bummer because there will be people that only play Wild. They have now gone ahead and changed this to displaying whichever your highest rank will be. This allows you to play either format and get the props you deserve from people on your friends list. This also allows both modes to be equally represented instead of Standard being the obvious top format of choice.
With the implementation of Standard and Wild formats on the horizon, we want to ensure that their addition to Hearthstone is a fun and meaningful experience for all our players around the world. Everyone should feel free to enjoy the format they have the most fun with, and be recognized for their efforts ascending the ladder in Ranked Play, regardless of format.
After careful consideration, we have decided to make a change to the rank displayed on your friends list once Standard and Wild formats go live.
Previously, we announced that your current rank in Standard would be the rank displayed next to your name on your friends list. Instead, once the Year of the Kraken officially begins, your best current rank–whether Standard or Wild–will automatically be displayed on your friends list.
For example, if you are rank 15 in Wild and rank 18 in Standard, your rank in Wild will be displayed.
We hope this change will encourage our players to proudly display the in-game accomplishments they have achieved, whether they are traversing Wild or diving into Standard.
[Source]
Last online ranking would be cool to show when friends are offline too and not only online.