If every city in the game was cross faction with one district in the city being faction only which district of which city would be limited to faction only?
I would imagine the keeps in Stormwind and Boralus would be faction only but the rest of the city would probably be open to Horde visitors.
Old Town is the most inhospitable district for Horde, although with recent changes the rest of Stormwind is catching up. If the rest of Stormwind were made friendly to Horde I’d be terribly sad, but I’d imagine either Old Town or the Keep would stay Alliance-only.
Orgrimmar is harder to say. I kind of think that the Troll area might, in some manner, still be unwelcoming to the humans.
The Undercity would rezone the toxic ooze canals to be friendly. For reasons. The same answer applies to Ironforge and their lava.
Generally, military sites or centers of government would make the most sense to me, since I imagine one wouldn’t want to give their enemies, former or otherwise, free access to such places even in times of relative peace. So, wherever the racial leaders reside, and places like the aforementioned Old Town in Stormwind or the academy in Boralus.