I think the Xindi would be cool if tied in with a Delphic sector but a lot of the plot surrounding the Iconians treads on the turf that a Sphere builder FE or even just one-off mission would cover. Uber-powerful race returning and all that, so it wouldn't really help STO to get a head start on the conflict the Enterprise J is involved in when, in the first place, we're only at F.
Wasn't that Alternative future only possible when there was still an Expanse (which was rendered moot when the spheres were destroyed)
There is one. The original captain of your ship is a Denobulan. He dies off-camera about six minutes into the tutorial, though.
As for the Xindi, a lot of the first set of ultra rare doffs were Xindi. All the surviving races are covered, including aquatics. Which raises the question of where on a small ship do you find space for the tank and attendant life support systems to provide living and working space to an inconveniently large whale-person?
First, I think a playable Denobulan (rather than just the Alien created one) would be fun. I think it'd have to be free though, not sure how many would pay for it. But that inflatable-face thing could be a great way to dump aggro (or a "confuse effect")
As for Xindi: Any of the humanoids would be a great addition. I'd pay good money to play a Xindi insectoid, even though I bet designing them would be a royal pain. I could live with Reptilian