Maybe the inn lies at a major crossroads, connecting two or more major civilizations. This would help explain the presence of people of diverse backgrounds. I suppose it could either be within a large city, or perhaps in a more rural area. Either way, it should probably have something special about it to make it worthwhile for (presumably) adventuring types to drop by. Maybe it's a particularly nice inn, or maybe it lies rather near a person or place of interest- the local lord is known for hiring professional 'troubleshooters,' thus attracting mercenaries, or a significant ruinous complex lies nearby, and adventurers, merchants, and scholars of all stripe have been drawn to the area. There might be illegal gambling in the back, or else the inn acts as a front for some other illicit activity. Maybe the players are in on it- or maybe they're trying to stamp it out.

Maybe the inn was founded by a famous adventurer, and he retains a reputation for being helpful to those still active in his former craft. Perhaps there is something magical or supernatural about the place- the inn acts as a conduit of divine or arcane energy, or maybe it stands on a site where the boundaries between various realities are unusually weak.

Just some thoughts. Oh, and I'd just like to go on record as saying I'm not especially fond of the idea of OP characters. But that's just me.