Sequels usually suck & tarnish the legacy of a great film, but I believe there is enough of a calling for a MG2 & with Tina Fey on board you can't go wrong.
To the people saying the actors are now "too old", well it most likely won't be based in a school-setting. I mean, a Mean Girls in the workplace sounds cool to me. Cady would've just landed the job of her dreams at a TV station, only to find out Regina is in bed with her boss & his PA is Gretchen-Big-Mouth-Wieners.
It could happen, Karen is Amanda's signature role, The girl who played Gretchen is doing christmas TV movies now, Regina is Rachel's signature role as well. They'd all say yes.
As much as I love her and Mean Girls, I don't want this. Not with that face of hers, at least. Sure, it's partly due to drug abuse, but the Botox and injections didn't help at all.
It would probably be a mess anyway. Sequels that long after the original just don't work. It would be some That's So Raven 2 ********, where the original stars take on secondary roles.