Tyranids start game on ground, respawns from flying carrier creatures. When they are being pressed into their lair, they spawn from spawning pools instead.
I like this idea.
Keeping true it the fluff, only one race should ever spawn in 'pods in every map, and that would be the race that has spacecraft superiority on that side of the planet.
I agree, although there might be a few scenarios where it is imaginable.
Generally, did we discuss "wave respawns" yet? Is that the plan for the respawn system?, I'm a bit out of the loop...
The way I imagine it it could be a very effective mechanism against spawn camping. Troops spawning in the dropship while its en route would enforce a wave mechanism. When you're pushed back into your base /lair individual respawns make sense as well (better chance to react to threats).
As to vehicles: They're tricky, and for least frustration I would limit them to static or scripted roles.
For example, you could have a map where some vehicle killer bio mechanism holds a bridge. Once that has been destroyed a convoy of vehicles (AI) could proceed over the bridge and take the control point there, effectively securing it for the Marines. The Tyranids could have similar "monstrous creatures". To keep it simple, however, I'd keep any vehicles as static spawn protection at the most!