There haven't been to many threads on what mortal races will be considered default for Urban Planescape. While theoretically, all races are included in the setting, certain species get around better than others. This thread will discuss which races will be included in whatever the Planewalker crew ultimately produces.
Here's a preliminary list of default races (races from the 2e box set are in italics):
-Human
-Aasimar
-Bariuar
-Gensai
-Githzerai
-Githyanki
-Half-elf*
-Modron, rouge
-Tiefling
Other races will be added to the list below, per the suggestion of other forumesrs.
), elf, half orc just for the sake of completeness and/or prime immigration
Though I'd have to look oddly at anyone who wanted to play an imp starting out. 

We'd also need the exemplars. Granted, most of 'em could probably come from the Menace and Monster Manuals...
So, we'll need to probably have any of the creatures like the 'loths that aren't likely to be featured elsewhere too.