A really good feature: If the user could subscribe to the forum and also to all topics, existent and new ones. This way the user will receive an e-mail for every post on a forum.

This would be really good for my users...

Yet also bad for those that do not want that many e-mails. Or an e-mail EVERY time a new post or topic made in (the) forum.

The user should be able to choose! It would be a type of subscription, but the existing ones would remains as it is :)