What I'd like to see in Advance Wars is two lines of games. One following the first kind of story line, and one following the Days of Ruin line.
I agree that they need to change things up. I think there's a lot they could do in the way of adding more kinds of infantry. They should also add more things that change up the terrain, like maybe infantry that can dig trenches or apc's that can lay land mines.
Another thing I'd like to see is different levels of units. They've done it well with tanks, but I'd like to see something in the way of artillary or rockets that can dig in to their positions, gaining offensive and defensive bonuses.
I also think it would be neat if they did something along the lines of fire emblem where you keep units throughout the whole game. It would be neat if you could name the units too. So you could have your "101st AIrborne" Bomber unit kick black hole all across the map.
Those are just some of my ideas.