There are a lot of characters out there who aren't really the age they look for various reasons. I'd love to see some of them team up for whatever reason - fighting time sensative aliens/demons, maybe?

Dawn Summers, BtVS: Interdimensional energy in the form of a human girl.
Connor Angel/Reilly, Angel: Technically is the right age, but grew up in a hell dimension where time runs faster, so was only born three or four years ago.
Luke and Sky Smith, Sarah Jane Adventures: Both genetically engineered by aliens to bring about the end of the world. Neither have done it yet.
Conn...

Fanon has it that in the future, Chris was involved in the Resistence against Wyatt. What is in that future, he worked with some of the Scoobies? When he comes back in time he recruits the younger versions of those Scoobies to help him protect Wyatt. Hopefully using the title line, since he figures they'd recognise it. Cue lots of Scoobies and Halliwells fighting each other and/or each other's bad guys. Set it in any part of Season Six Charmed and anywhen in Buffy.

I don't have any 'must haves' but if it involved Dawn, Connor or Andrew - great. Use any combination of Halliwells, Scoob...