If X-Wing was, well... X-Wing the game and Armada was Star Destroyer: The Game, I expect this to be AT-AT: The Game.
I'm gonna expect it to play, balance-wise, similar to Armada now. Empire's going to rely on a lot of heavy armor units with disposable infantry (so Hev/medium units with expendable "fighters"), while the Rebels get a few purpose-built armor units (like the speeders), but are packed with all kinds of crazy infantry types that all love hiding in terrain and sneaking around.
If FFG's not careful Legion is going to end up like Armada now: People love the small units that are hard to hit but hurt way out of proportion to their size when activated en-masse (plus deployment and activation advantages, depending). If there's no particular reason to take Armor (like you're not always demolishing bases), people will default to Infantry and fast speeders in skirmish play because they are harder to hit and do more with less.
Though as I think about it... the Rebels had to get creative because AT-ATs were nigh invincible against infantry right? That's more than we can say for the poor ISD being mobbed by fighters here in Armada.
Edited by Norsehound