I love ST2 and look forward to paying for the upgrade to ST3. That having been said I think it could be a lot more future proof:
I have several ARM devices in addition to x84/x64 devices and I currently can't use ST on any of them. This includes a jailbroken Surface RT and a Exynos Chromebook. It would be great to be able to use ST on these platforms... at the moment I have to maintain ST and VIM settings on all of my devices to switch back and forth between.
Secondly, support GTK3 seems like an obvious feature. It's pretty much everywhere that matters now, it would give us proper smooth scrolling, and presuming you don't make additional x11lib calls by hand, it would give you Wayland compatibility for nearly free.
Thirdly, if GTK3 isn't enough, Wayland is upon us! It'd be awesome to have a Wayland compatible build of ST.