Why would Vulcan leave the UFP? But that is probably a discussion for another board.They could have used Dr Selar (TNG 'The Shizoid Man') if they writers had gone through with the idea of Vulcan leavin' the Federation instead of makin' the Klingons bad guys.
Troi would have fit in easily, seeing as they didn't have a counsellor (until S7). She might also have gotten to explore her new Commander rank in a little more depth.
The rest of the main cast would make the actual DS9 crew redundant, as they already have a CO, XO, Engineer, Doctor and scientist. Worf was probably the best candidate, as he could slot in, bring more action to the series as well as the Klingons (which really wasn't needed IMHO).
As for recurring stars:
Barclay - I always found him something of a 'meh' character, didn't dislike him but he wasn't on my list of favourites either.
Ogawa - A nurse for Julian to harass?
Ro - She could have come on quite nicely. her 'defection' to the Maquis could have been part of an Intel plot to gather more intelligence.
Guinan - Opens up a cosy little bar, where she offers advice and an open ear. Someone to threaten Quark's business and for Odo to be more suspicious about.