I love Advance Wars. It's one of my favorite series of all time, and made the almighty GBA even more awesome. I have an idea for a Wii U entry, and so here goes:
Create-A-CO: Create your own CO. Customize his or her looks, proficiencies, and powers.
Online 4-Player Battles: Pretty self-explanatory. To avoid 3-hour sessions, there may be Words With Friends-esque "at your own pace" multiplayer in that you take your turn, the other players take their turns and you get a notification when it's your turn again while you do something else (probably if this was a downloadable game)
New Controller Utilization: Your TV would act as a live view of the complete battlefield and give in-depth stats, and you would carry out orders on the touchscreen. This would prevent other players from seeing your actions in splitscreen.
These are just a few ideas that I have. Personally, if this was available as Wii U Ware, I'd buy it in less than a heartbeat.
Since the Advance Wars series' has only been on handhelds, I'd imagine that the next installment would be on the 3DS.
If they make Advance Wars 3DS (and make it a successor to Black Hole Rising or Dual Strike rather than the disappointing Days of Ruin), I may be tempted to buy a 3DS