If you're on a demiplane owned by a trickster deity, you might think about how that relates to clerics receiving their spells. If the trickster deity interferes, then other divine spellcasting classes or other characters with healing abilities, such as oracles. Alchemists and bards seem to lend themselves to urban campaigns too.
If "allying with powerful entities" includes obscure, largely immaterial spirits you must bind yourself to (akin to the 3.5 Tome of Magic vestiges), then you might look at Radiance House's Pact Magic Unbound stuff (which is quite similar.) The books aren't specific on how much/which spirits a pact-magic user knows about, so discovery and mystery could very much be a thin if you wanted it to be.
The biggest thing I think you'll run into is what to do with "wild" characters like the barbarian, druid, and ranger. The barbarian and the ranger are fairly easy to refluff for urban settings The former being madmen, thuggish gang members, REALLY spoiled aristocrats, etc. and the former being bounty hunters, sewer explorers, etc. Druids are a little bit more challenging. Here's some archetypes that might be appropriate for an urban game:
Advanced Players Guide:
Druid archetypes: Urban Druid, (Possibly) Blight druid
Ranger archetypes: Skirmisher, Urban Ranger
Barbarian Archetypes: Armored Hulk, Urban Barbarian
Druid archetype: Menhir savant (provided the demiplane has leylines), Reincarnated druid (especially if souls can't leave your demiplane)