As long as I don't have to switch over to those new home, back, multitasking buttons I'll be fairly happy. To hazard a guess, HTC will continue to evolve the UI, but they're not going to switch over to stock suddenly when L comes out.
Non HTC apps will be pure material design whenever they're updated though. You can get a look at this a bit with the newest play store update. It's still rolling out but you can always push the apk manually. Made the pages for individual apps much more material designy.
Sent from my HTC One M8