The default setting for D&D (and Pathfinder) is medieval, and it's a fantasy world we all know pretty well, and understand.
A few years ago, I got to play in a Roman-influenced game, and it was a lot of fun. We fought undead in the north, surrounded by Gaul-like people, then headed back south to the capital, then crossed an ocean towards the Persian-style place. I'm told there was even room for some Greek island hopping, which we never got around to.
I've been daydreaming for about a year about my own Roman setting, and how I would do it. Let's call it Republic for now. It would be my first foray in Pathfinder if I ran it, because I'd like to see how the play is different to D&D 3/3.5, and I've never actually sat down to it before.
Since Republic isn't Rome, but Rome-influenced fantasy, we still need to cover the bases. Let's start with classes. The four base classes are pretty easy to cover.
Fighters could be former Legionaries, or some form of auxiliary, or what have you.
Clerics might need to be renamed as 'Priests'. We'll argue later over whether we run with Christian Rome as the influence or not later.
Rogues would work, but would likely be made up of a lot of commoners, maybe indentured slaves (escaped or otherwise).
Wizard is where it gets tricky. I like the idea of scholars, but I don't know how well Wizard fits. A scholarly Sorcerer would work just as well (though maybe the influence of that old game is at work here, since that's how the setting worked).
Of the other classes, Druids and Barbarians fit, filling the roles of Fighter and Cleric. Monk and Paladin have less of a fit, Bard and Sorcerer are more of a variant of how the Rogue and Wizard might play. I'm still fiddling details in my head. Ranger would be the equivalent of scouts in the Legions, or might be woodsmen and hunters, and certainly fit well with the Druid and Barbarian mentioned above too.
The only other things that might fit are Pathfinder's Oracle (inspired by Rome after all), possibly the Alchemist, Magus and Witch. I'm still not sure if they're really needed.
Ok, now for some trickier parts - PC Races.
I'm pretty sure that the Dwarfs as trade partners to the north would fit pretty well, and something about Elves being some kind of Sylph would be interesting (it's what I'm already doing with my Kingsmead setting after all). Gnomes and Halflings seem like they'd be an easy fit too, though might be best expressed as more urban/less rural cousinds of the Dwarves and Elves.
Do the Half Orc and Half Elf races fit? At a push. Again, more to twiddle there.
After that, limiting equipment is all that really takes precedence. I've got some ideas for where and why the PCs would adventure, and who doesn't prefer the idea of the PCs meeting in a taverna, rather than a pub? Right?