You don't have to request or offer those tags at all. If you want to receive/create fanworks with a worldbuilding focus which also feature canon characters, then you will absolutely be able to sign up accordingly.

However, because it's possible some people will want to request/offer things without canon characters (especially in the "Meta" category), I want to be able to accommodate that. So even if you're not planning to use those tags yourself when you sign up, it would be really helpful if you could nominate them for the benefit of other fans of the same fandom.