Will Apple release a new iMovie for Yosemite?
Good (and tough) question. No announcements made about this and the iLife/productivity apps-cycle is unpredictable, so I'm just going to speculate. I think its safe to assume iMovie will get updated alongside the other iLife/productivity apps, just like in the past. Bringing a visual overhaul to all of these apps would take a lot of work, but it would also a great opportunity to make other changes/re-imagine already existing features.
I'm guessing we might see a small technical update to the apps around the same time they're releasing Photos (early next year?), but I think we might have to wait until fall 2015 and OSX 10.11 for the next major release of these apps.
EDIT: Oh, and just another thing worth thinking about. With iPhoto merging into Photos, I guess this could be a possible direction for iMovie and the Video-app on iOS. Could take alot of work, so maybe we'll hear a similar announcement on the next WWDC and a possible release early 2016.