I would VERY much like to see some form of anti spam measurement added into the core, beyond just any form of captcha; which is often easy for bots to pass through, yet hard for real people who want to join.
Spam bots have become so good, at email validation they can do in their sleep... If they slept, which is a problem, they don't, but you and your staff do.
I would like to check Stop Forum Spam (StopForumSpame.com) against known email addresses, since IP changes happen faster than new email addresses. Of course IP checks also be a valid option check, though I would like the option between one or both.
Akismet (akismet.com) is also something I would like to see incorporated into the core registration. This has become the default in WordPress, but also many alternative addons.
I stress this need because recently, the ONLY anti spam add on for Woltlab has been discontinued. This leaves a large void, but further stresses the point on why such an important feature should not be left to chance.