An at- st. It shoots either a bunch of shots or two big huge power salvos depending on weather you tap or hold the ability.
AT-PT
first mech that doesn't rotate, and turns slower, but it is fast and has a faster boosted movement sprint. It also shoots, but not too powerfully, just anti infantry fire that can do enough damage over time. This would be the ability.
Instead of a gun, maybe some swords or hardlight sheilds (like the metal sheilds- front only and energy) or maybe some energy sheilding. The sheild would be like having free healing or helth but not quite, and the sword would fling you forward for some reason like halo.
mech
An army of infantry With rocket launchers.
A spartan unit that can take over mechs and eject and do like no damage and is really small and maybe slow.
A seige tank mech that locks down for mor or different samage.
the Thor from starcraft would be great. It stops all movement and other weapons and launches an artillary barage in an area.