I'm A Banana
Potassium-Rich
Aasimar
Bariaur
Githzerai
Genasi (air, earth, water, fire)
Exiled Modron (maybe?)
Human
Tiefling
Yup, that'd be my classic list, too. I'd be tempted to add a law-touched and chaos-touched group, too, but it probably wouldn't be necessary at release (for that, grab the Planes of Chaos and Planes of Law expansions!).

Though I think we could probably add all the base D&D classes without much conflict, too (perhaps with "urban" or "planar" variants from UA applied): Barbarian Xaositects who roam around executing vandalism, druids tending isolated Sigilian gardens in the Lady's Ward, etc.
And, yeah, expansions could add more without really ruining much for those that didn't have the expansions: there'd still be all the old portals to explore, the expansions would just give you "new" ones (or at least new regions).
Fer'instance, you could add Githyanki as a playable ECL 0 race, along with some psionic races and a lot of Astral locations by buying the "Silver Sea" expansion. But if you never did, you could stillk go on adventures everywhere else and be every kind of character you like.
I'd recommend going light on the cant, just a bit for flavor ("'ey, berk!" instead of "Hello", "Scrag 'em!" instead of "Get 'em!"...and only for native Sigilians).