I am currently running a campaign that is coming to a close.  This is fine, and as planned. However I would like to start another campaign after. I mentioned this to (some of) the players, who seemed happy(ish) to switch over when the conclusion was reached, including that I was considering a vaguely Gothic-horror theme.  I also specifically stated that there would be a session 0 to discuss things like setting and appropriate characters.

Having heard this a couple of players have, I discovered, started creating characters, of varying levels of appropriateness. I have tried subtly suggesting that those may require revision but at least one player seems attached already.

My question then is:
 **At session 0 how can I persuade players to be more willing to abandon (potentially campaign inappropriate) character concepts, especially ones they already seem somewhat attached to?** 

Ideally without causing offence. If it would be better for me to change my plans, I might accept that as an answer.

**Edit For Additional Detail:** The character is a pacifist illusion wizard, and I am trying for a setting with unavoidable violence (mostly against monsters) and 
extremely rare and distrusted magic (so wizard would be exceptional enough to be disruptive). 5th Edition d&d, but I don't think that's relevant.