And I can't really imagine the Glee crew passing up the opportunity to raise her star higher.
If she's graduating at the end of next season, her star will be of little concern to them (though I'm not convinced that franchise is really done with Lea or a bunch of the others).
They accomodated her doing New Year's Eve
earlier this season (why, Lea, why?), but that was only a few days' work and didn't entail missing any episodes. This would be a much bigger commitment.
In other casting speculations, after Daniel Radcliffe's success with How To Succeed In Business Without Really Trying
, he might be a good choice for one of the younger male parts.