I know this thread is a little bit old, but a friend of mine and me have been thinking about this exact same thing for years now.
Well, I started building something in Unreal Engine - it is far from done and doesn't look like JK2 at all (yet). I have stopped working on that for personal reasons but I thought I still might share this with you.
Maybe some people with programming experience would want to pick this up. It really is just the start of a larger project though. Basically I worked on networking like client side interpolation and prediction to cope with lag, some basic animation and hit box handling, some bunny hopping and so on.
If there would be people willing to work on this I would most likely start developing again. Especially 3D assets were a big problem for me.