I think a great new feature would be annual stars or other images that automatically get awarded to members on the anniversary of joining.
That can be interesting as a MOD, but for sure not a core feature, methinks ;)
There is this: http://custom.simplemachines.org/mods/index.php?mod=1915
Which could probably be tweaked to calculate the time from registration instead of time online.
I have a paid addon called badge awards http://www.smfhacks.com/badgeawards.php that awards badges on registration anniversary for each year up to 10 years by default