Also, about branching. I don't think you'd really have to maintain two projects. Classic klystrack would become pretty dormant. I mean, it's in a very usable state, only very rarely crashes, and is pretty much a complete package as it is. You'd only really need to come back to it if someone discovers a major showstopping bug. Branching off into a Pro version would enable you to not worry about backwards compatibility at all and change entire sections.
I think I'll try to finish 1.6 and then keep that as some kind of basic version and everything after that could be thought as "deluxe" version. Every version has brought more and more features anyways. The change however large or small will be incremental due to the amount of work anyways. That basically means if you don't count some kind of overhaul of the GUI (mainly larger resolution support), meaning a rewrite of the layouts etc., it will be pretty much how the project advances now.
There's always a risk when making a total rewrite or some very large change in one step, that someone gets bored or fed up and then quits and there is no working state. With incremental changes, even if some accident happens and all source code is lost (e.g. Google engineer spills beer all over the server farm), there would be a working, improved version released at some point. And it's much more motivating to everyone if there's a trickle of new stuff that is pretty much ready to use.