I want Odo to come back and lead an invasion of the AQ then DQ and be the supreme bad guy. I want him to make the Female SS look like Kai Opaka.
Actually, I think Odo coming back as the big bad with some kind of believable motivation would be quite an interesting concept.
Far better to leave DSN as it is, after all too much of anything isn't always a good thing. In other words more might be rubbish diminishing what we had.
So make it DS9's answer to Voyager's Fury only have it last the entire series? Isn't that what we're talking about here?
Let the new big conquest come from the man who knows us best and upon full consideration with his entire species decides they really are the better administrators of peace and order for everybody. His symbiotic Great Link ocean race and the celestial Ascendents should make for interesting villains. I want to see Thirishar and Family getting it on in four gender procreation while we're at it, and a stunning new station be emplaced for Bajor's thousand year golden age upon the Federation's arrival