At this point, the franchise has picked up so many characters or aliens or plot threads that started as one-off gimmicks, then developed them into rich, long-running elements of the universe. I must admit to being baffled by the insistence that because it's not 100% all the way fully developed in the first episode, it never could be, given how many times we've seen that position disproved at this point. I already mentioned the Barzan (who really set the bar -- if aliens as insubstantial and forgettable as the Barzan can be resurrected in an interesting fashion, what are we even talking about?). Just as implausibly, we've also seen the Pakleds turned into real adversaries. The exocomps were props, and now we have an exocomp that has had a great multi-season arc. There was nothing to M'Benga, and he's become fascinating. Or Christine Chapel - based on what we saw in TOS, there is nothing that warrants further exploration, and yet she's a standout on SNW.
Or going further back -- there's not much interesting about the Cardassians of "The Wounded", beyond their visual design. "The Host" leaves nowhere to go with the Trill. One of the best parts of Enterprise was it's exploration of the Andorian's, who were nothing more than intriguing makeup when ENT picked them up.
Obviously, if no one working on the show ever gets inspired by the Sheliak, they shouldn't force themselves, the Sheliak are not a must-revisit item. But there's plenty in a non-humanoid race with a cool design, a distinctive pattern of speech, and a unique value set to potentially spark the imagination. There's interesting dramatic tension in the fact that they regard humanoids as vermin, yet do hold themselves to a strict code in their dealings with them regardless. It certainly has more potential than yet another race of warriors or hunters.
I mean, the Overlookers on Voyager were all about bureaucracy, not an obvious avenue for drama. Yet they made awesome adversaries, because of clever writing. The stories with them had a freshness that other DQ adversaries lacked, because they were coming at it from a different angle. The same thing could potentially be done with the Sheliak.
