A ground vehicle would be fun, but we've already seen what can happen when a new variant of combat is introduced.
I'm talking, of course, about shuttles. Shuttles use different groups of enemies, as they can't handle normal multiple-frigate groups. Ground vehicles would presumably use new enemies, or a larger number of normal ground enemies. Shuttles also use normal space maps, but ground vehicles would pretty much require maps built just for their use.
Anyway, shuttle content has been slow coming, partly because before this season you had to change out at a starbase before using them at all. Of all the missions in the game, only 3 are for shuttles: the Vault, Vault defense, and Operation Gamma. Even without fundamental challenges on the maps themselves, making missions for a somewhat 'novelty' content type is slow. Normal ground and space content will always get priority.
Not that these reasons would prohibit ground vehicles or missions made for them, but it shows that any such attempt would likely be the only content of that type for quite a while. So I wouldn't expect a whole 'third' combat type any time soon.