I've been thinking about this a lot in making my X-Men fancast... I finally settled on Ellen Wong after seeing her as Knives Chau in the Scott Pilgrim movie. She reminded me of Jubilee in that movie lol. Plus she's young (and looks even younger) and fun and fit enough to do action.
eta: Sorry if this has been posted - I searched the forum but didn't see this post, and it didn't pop up when I searched, even though I assumed there would be one (seems like there is nearly every character's forum). So, yeah, apologies if I just missed it!