I'm not sure all of those folk would team together right away. The Drow might very well fall in with the fiendish forces due to their ties with Lloth, as might the chromatics with their ties to Tiamat.
Gith and Illithid might call a momentary armistice but maybe not. Maybe one of the PC's jobs will be to broker a piece between these two ancient enemies. A diplomatic mission to the Liche Queen to beg her to call of her hoardes would be nifty. But what about the Liche Queen's tie to Red Dragons? Would they ask her to go with Tiamat and the Fiendish forces? Interesting and complicated planar political stuff. Neat.
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Hmmm... would even the evil gods side with the Fiends? I think not. For the gods, their very survival and power comes from the prayers of their faithful. From what I've read in the Manual of the Planes and in the BOVD souls that demons and devils get their hands on go to the ArchDevils and Demon Princes. As such, those souls do NOT go to their gods. If the fiends launch a war on the prime and all of the evil gods followers are getting killed then the gods begin to lose status. Isn't that the same reason the the gods in Scarred Lands fought the Titans? Even the evil gods fought the Titans in that setting.
As far as the Gith/Illithid alliance goes I'd say that as long as the Lich Queen is around that it wouldn't happen. Of course you could launch a great adventure around destroying her and putting a more tolerant ruler of the Githyanki on the throne. The Githyanki would for the first time be allowed to worship a god rather than the Lich queen. All that additional power coming from new worshipers could give the gods a fighting chance against the fiends. That could be the turning point of the whole war.