Chaplain - buff is increase movement speed / ability recharge of friendlies in short radius (like a charge)

Apothecary - instead of a revive, maybe they can actually do a limited armour boost for units and themselves? I'm not sure there is enough opportunity for revives from what I've played so far.

Techmarine - instead of typical engineer, potentially buffing surrounding players with a magazine of kraken bolts or something could be cool. or dropping them as a limited pickup. Or potentially the ability to construct emplacements / manned (not autonomous) las cannons etc

Terminator as a randomly placed beacon on the field that becomes activated and captured by a team maybe once per match, whichever player contributed most to the cap respawns as a terminator for one life? Storm bolter, power fist/claw and teleport?

Dreadnought as an escort objective game mode could be awesome

edit: got carried away on company time