The lives of vehicles are hard and fast. The Sith not only need a rancor (which would be TOTALLY awesome), but the death of a dragon should definitely be permanent (or at least until the end of the game). I'm not sure how you want to balance summoning, but "whenever the fuck I want" takes a lot of the challenge out of the game.
Once the dragon dies, its gone for good. The ninjas only get one dragon summon per game, and they can only summon a dragon if they have their DX outfit (the one with the dragon on it). I would like to build a rancor (and have tried in the past), but I have had trouble trying to get a good design. I looked at ACPin's version on MOC pages but that was far too difficult for me and I don't have the pieces. As for the vehicles, I love vehicular combat, and like seeing large columns of tanks blowing the crap out of each other, thus I load my tanks with powerful weaponry and although most of them have pretty decent armor they have a tendency to get utterly destroyed in an average time frame of about 2-4 rounds.
What about a rival team of ninjas (Cobra Kai?) or those snake guys, and they can call in heavy support too? Or maybe the sith get an airstrike or something capable of equal damage to a dragon to play with whenever a dragon is summoned?
That is a good idea. I don't have any of the new snake people yet though so I've just used my skullkin as skeleton ninjas, and their vehicles (the turbo shredder and that helicopter that came with my lightning dragon) are the only skelly vehicles I have. Since they have almost no weapons, I just give them supernatural dice and say that they can shoot dark energy balls from their mouths or whatever.