Custom space replacer

[soshimee]

I would suggest a custom space replacer. For example, you could set it to -, for better distinguishing between actual underscores and spaces.

[Tim203]

I actually quite like the idea of using another invalid username char like - to ensure the uniqueness of the username, and making this the default value.
The only potential problem I see with it is that for username filters, . has been exempt from some of them because some Java usernames actually have a dot in them. However, I don't believe that - has that.
However, since 3.0 is a breaking release it might be the best moment to introduce it.