I don't understand how you think this would work unless the host changed every few episodes. Could you give an example of the sor of story you mean? A description in a few sentences?
What I mean is that the Trill species (the slugs) or certain Trill individuals (again slugs) might have had an agenda involving some infiltrating, and the unpredictability in the choice of host species would mean our heroes would never know if a person they interacted with was in fact part of the Trill agenda.
The Trills could be an actual threat (they want Starfleet out of Bajor?), or then more like comedic relief (this luscious Trill is out to lure our young men & women to the slippery slope?), or anything in between (what face does this notorious Trill bank robber wear?).
The concept of changing faces every now and then would have worked against the background of "The Host", in which a Trill did change faces twice. The concept of one and the same host staying till its life is over would also have allowed for the above plots, too, however.
AND for the Trill Klingon. Although I think Jadzia was a fun take at that very thing...
I see what you mean, and it has a lot of potential. But for it to work, the Trill would need to be able to change hosts a great deal more often than what we saw.