I guess it's mostly that the Tholians have acquired at least SOME degree of flavor because of their popularity as beta-canon antagonists. The Sheliak... not so much, as far as I can tell. Bringing them in doesn't bring in much new.
Actually, if it weren't for the schtick that the Tholians live in a very incompatible environment with humans, whereas the Sheliak coveted a human colony world... there's no fundamental reason the Sheliak couldn't BE the Tholians.
I don't think the Romulans are ready for that. They're still the same empire that in canon held to a near-hateful paranoia for another fifty years. They are only very slowly learning to trust the Federation.
I don't know if the Romulans can feel confident with the borders without a neutral zone. At the least, I doubt they can learn to be good neighbors with us until some time after the Romulans whose formative experiences were the Earth-Romulan War and its immediate aftermath have died. And all those who are in senior positions are those who were at most mid-rankers during the Biophage Crisis.
Which, given Romulan longevity, is going to be some time in the late 24th century...
Yeah, but Oneiros also seems quite capable of making up his own species (Amarki, Gaeni, Apiata, Dawiar), and we the playerbase have detailed out some others (like the Yrillians) that we're having a lot of fun with. I mean, would you really want to trade away the Apiata, whom we have come to know and like, in exchange for the Bolians or something? Give up the Amarki for the Denobulans?