I'm just starting a Forgotten Realms Unapproachable East campaign, and I'm allowing any base class that fits a character's background. For instance, Samurai, Ninja, Shugenja and Wu Jen are fine for Shou Expatriates, Spirit Shamans fit right in among the Rashemi and Tuigan, Swashbucklers are naturals for the cities of Thesk and Aglarond, Hexblades could be members of elite units under the command of Thayan Tharchions or Zulkirs, Scouts aren't bad additions to Aglarond or The Great Dale, Favored Souls can fit about anywhere, Marshals could lead Aglarondan, Rashemi, or Thayan armies, and Healers could work in Aglarond or among the elves... the only new classes that don't really seem to have a comfortable niche (from the new books) are the Battlemage, Warlock, or Spellthief.
But that's just core- that's not even considering Unearthed Arcana variants, Arcana Unearthed classes (most of which I would prohibit), FFG Path Book variants (most of which could fit)...
I have no problem allowing new core or prestige classes, as long as they seem flavor/background appropriate...