I've seen a fair few posts of people saying that the Rebels don't have any heavier stuff to pull from so I went ahead and made this list below to show what they could get:
A-A5, probably comparable to the Imperial troop transport we see in Rebels.
AAC-1 Speeder Tank , rocket launching tank.
AT-PT , if they aren't given to the Empire.
Artillery , looks like it could easily be a movable anti-troop turret.
AT-AP , probably a tad too big for the game's scale.
BARC Speeder , allows for something different than the speeder the empire has though kind of takes away from what a Republic faction might have if they ever add one.
Droideka Oppressor and Sentinel , again movable turrets but at the same time thanks to Commander being canon (or at least what shows up in it rather than story) they are in canon now.
Endor Forest Ranger , kinda meh looking but is canon.
Freerunner , repulsor tank and is canon.
Juggernaut , goes either way with Rebel or Imperial but I think it is a tad on the too big scale for the game. There is a better variant for the rebels but we've seen it used by the Empire in Rogue One so feel it leans more that way.
Heavy Tracker , a rebel tank from the WEG RPG days.
HTT-26 Transport , rebel troop transport.
Hailfire Droid , rocket launcher droid though takes away from the CIS if they are added as a faction down the line.
ISP , anti-infantry speeder but again possible Republic vehicle if they are added.
LAAT/i , more possible Republic fare and yet another troop transport option.
MLC-3 light tank , light tank that is canon.
MPTL-2a , movable heavy artillery.
Rebel Combat Speeder , kind of basic but works for a simple tank.
T-47 , if the size allows for it then it is a pretty obvious choice.
T-1B hover tank , simple repulsor tank from Rebellion.
T-2B repulsor tank , a repulsor tank that is more well known due to Empire at War and also is canon.
T-3B Heavy Attack Tank , kind of in the name.
T-4B Heavy tank , more well known heavy tank.
ULAV , canon rebel landspeeder with blasters.