Well, as evidenced by ESB and Rogue One, the only thing that we KNOW will be in the game that can damage an AT-AT is a lightsaber (and then it can only make small, oddly rectangular holes in the underside). Blasters? Nothing. Infantry scale rocket launchers? Nothing. Snow Speeder blasters? Nothing. We have seen AT-ATs destroyed by the following things in canon:
1) lightsaber plus big bag of bombs
2) tripping them and exposing their oddly placed "explode button" to snowspeeders
3) oddly powerful ion blasters used as door guns
4) X-Wings
5) a lucky force guided shot from a Clone Wars era AT-TE
6) another AT-AT
If we count Battlefront as a canon source, we can add "massed firepower and bombing runs from Y-Wings"
So really, my issue isn't "can they do it" or "will they do it" but "should they do it". I'm not entirely sure they should. If they put in an AT-AT, they have to give the Rebels something that can kill it, and even if they do an "epic" format, if even one of the AT-AT killing options is legal for standard play, it would run the risk of obliterating other vehicles and start the power creep process.
Now I'm not saying one way or the other. Having AT-ATs was never my core desire for this game, so I don't care one way or the other. I just hope that if it does happen it will be done well.