I'm very excisted about private Beta too. Can I join?
My nr one added feature would be to have nested symbols. This would be a game changer for me as the combination of the balsamiq-type overrides with nested symbols would beat any wireframing tool I'm using today. Sketch is doing well in this area, but are still kind of simple in the overriding functionality of symbol instances.
A bridge to interactive prototyping tools would be nice, but then again, I use Balsamiq as a (very) rapid mockuping/wireframing tool for the early stages of my design process. I'd love to see the same strategy as sketch has: for 3rd parties to be able to write modules on Balsamiq. This would immediately bridge the that gap towards prototyping tools. Well if a 3rd party / community member would write the module that is. If you guys open up your document format (like version 2 and like sketch has done) it would be even simpler to realize some of these bridges. Sorry, this last paragraph is like a 3 in 1 feature request
Hope I can join