Some of us have already begun modding. There is a LOT you can mess with, it's not worth waiting because everything you want to mod is already here.

HOWEVER, if you plan on doing any core modifications then you should probably wait. I believe the rebuilding of the core files into the data.zip is broken at the moment (you'll need my fix if you do it). We need a better way of overriding default core gamedata then we currently have.

The best thing to do now is start with the little stuff so you can focus on the bigger stuff later. Start make the Kingdoms, items, armors, weapons, and tiles now and have less to do later and release it earlier. No reason to wait, really.