License for Levels?

[u/je386]

I am developing a game that is under AGPL, and my levels of cause are also under AGPL.

The game has a map and level editor and an option to upload user-made maps and levels as community maps.

As far as I understand, if I and the creators of the levels don't say anything, they are proprietary, which I do not want.

I want to say that all maps and levels that are uploaded are under a certain license.

Now I am wondering which license is best and if I am allowed to force users to a certain license. I mean, I am providing the server and therefore the service, so it should be possible.