I was too late for my crackpot theories but Cass, while you have already used the 5 T-shirts I want to offer my dumbass ideas for your T-shirt Cannon!
It all revolves around the Jasper Prophecy.
The prophesy talks about an alternative timeline where you don't exist. Fern fights Meteor with Arc, Cal, Lin (not Anna, this is the crackpot part,), and Shade (!?)
Fern ultimately fails and Arceus comes to destroy and recreate the world, but Lin wishes for the world to be remade but with someone strong enough to stop Arceus after they're released. Lin could stop Meteor now, but she can't beat Arceus itself, so she needs to become a villain to create enough of a challenge for you to become strong enough to beat Arceus. Shade was in the original party but died or something before the timeline reset so she's spooky now (ok that one's a stretch but we don't have much to go on for Shade)
C'mon, a theory this nutty deserves to either get a cannon or to be canon, your call :)