I have seen discussions about the possibility of an AT-AT being introduced in a future wave.
To my knowledge, the rebels don't have a land vehicle large enough to be a good counterpart.
Infantry units such as heavy weapons specialists and long-range snipers seem almost like an inevitably to me, along with stationary/mounted guns like the E-Web.
If anyone can think of a rebel at-at counterpart or has any other speculative thoughts please reply below.