I must say that I think a Romulan Starfleet Captain is more plausable than most of your examples. I think the Kreetassan are very plausable, but we only ever see them in the 22nd century. They may or may not be a Federation aligned species: Very plausable. The Karemma are a Gamma quadrant species who appeared to be quite comfortable in their place within the Dominion so chances are: No. The Vaadwaur are native to the Delta quadrant and were hostile to Voyager. So, not only are they *really* far away, but I don't think they'd fit comfortably we even a Federation allied species. So: No IMO. You'd have a better chance with the Karemma. Wasn't Captain Rixx a Bolian? In fact wasn't he the first Bolian we ever saw? I'm sure you mean having a Bolian captain for a hypothetical show... but I'm just pointing out we've seen Bolian captains before. The Vorgons were unfamilar to Picard as of 2366 and the only examples of their species are a pair criminals from the 27th century, so I'd have to go out on a limb and say the Vorgons are unlikely for at least a hundred years. Denobulans are pretty plausable. It's just a question if a Denobulan would seek out command of a starship. Course, we haven't seen them on film since the 22nd century. The Suliban are pretty neutral last I checked. But weren't they rumored to be extinct by the 23-24th century?