I really like your proposed changes, the code definitely needs refactoring, I would be willing to aid you wether you decide to branch out or contribute to hxp3, if I can.
the thing I am most unsure with about 3.0 is the complete abandonment of BitmapData / blitting, if I understand it correctly. I still use a lot of the functionality like blendmodes, color and image manipulation (setPixel, getPixel) and bitmap texts.
To be honest I don't have any idea how for example I could realise something like procedurally generated level graphics for a game without flash (openfl) bitmap data.
Also since most of my games have a low-res pixel asthetic the move away from 2d might not be very beneficial for me. It sort of is a bit awkward already with sprite smoothing always on and no easy way to switch it off for the whole game.
Maybe shaders could become very useful but as of right now I can't find any documentation on how they work and how flexible they are and also they appear to be full screen only.
I really wish there would be a way to have the best of both worlds because 3d does seem to be an exciting move forward.
I think when 3.0 rolls around this site also needs a major update (and also stuff like logos and design baselines).