Thanks for your prompt reply, @luceos.
I fully understand your point, but at least in my project, I think that many users will refuse to create an account if they see that two factor authentication is enforced.
What I've learned through different past projects, is that users tend to accept quite easily the extra effort of two factor authentication when their data is in risk, but not when you are in trouble.
If someone breaks their password through brute-force attacks, the forum admin is who will have to deal with any potential spam. It is unlikely that attacks are against user's data, as not much confidential information from the user, apart from the email address, is in risk. Thus, I think that many potential users will simply prefer not to join if they see that will will have to run this extra mile to connect.
I felt a little bit surprised to see that with so many extensions available, none is covering this situation, as I thought that it would be a quite broad issue. I was even more surprised to see that even the extensions that warn that a password is weak, let the users shoot their own foot afterwards. I fully understand that most of the effort here is handled by volunteers, so there is not much I can do, but I admit that I expected that it was a much broader concern in the community.
Thank you, anyway. I really appreciate your time.