I think it would've been great for Geralt to be involved in a battle for Novigrad. It would've been particularly interesting, since the game does a good job of demonstrating that every side of the conflict was severely flawed (Nilfgaard is an aggressive conquerer, Radovid is crazy and basically evil, and Novigrad itself basically has state-organized pogroms). It would've been genuinely hard to determine who to side with, and the game could've easily shown some pretty bleak consequences of the victory of whichever side you supported.
This is actually possible in Reason of State and related side quests, even if the plot in these is not among the best done ones in the game. There are three possible outcomes to the war depending on the decisions made in the quests.
With the battle for Kaer Morhen, Walking by characters I really cared about, not knowing whether they would survive...that added enormous tension during the actual fight. I don't think that would be as possible with any battle in Novigrad. Dandelion, Priscilla, Zoltan, Triss, Dudu...those are the only characters I would really worry about in Novigrad. And only Zoltan and Triss would be likely to actually fight.