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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.

(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.)

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.

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.

(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.)

My question 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.

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.

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.

(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.)

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.