Everywhere and Nowhere

Deities and gods- 'the Powers' in Planescape-speak— hold a unique position in this campaign. They are apparent- their homes are literally on the planes- but they very rarely (perhaps never) make a direct appearance- especially in Sigil, where they lose their divine status. However, the realms and followers of the gods are major and significant features of planar exploration. Clerics and worshippers are somewhat rare… but petitioners- the souls and spirits of mortals who have died and whose bodies reformed on the plane that matches their previous alignment or devotion- are quite common.

Which Deities Should I Choose

Please avoid the exclusive Forgotten Realms Gods (like Kelemvor) and the Eberron gods entirely. In general most of the FR gods are ok, but because of the different cosmology (ie not the Great Wheel) and the fact that much of Faeruns gods come from mortal origins (and seem to get killed and supplanted quite a lot) they aren't ideal. All of the Humanoid gods are ok. There's more diversity out there: All of the 'World' deities from 1st edition AD&D (this includes Egyptian, Norse, Greek, Chinese, Babylonian, etc) are on the table, as well as any number of the 'humanoid pantheons' (the extended orc, elf, dwarf, etc pantheons..)