Rather than version control as such, when our group writes at the same time (we use latex), we use SubEthaEdit to write actually collaboratively. It's a serious step up from version control. Requires a little more trust, but that's fair enough in co-writers.

The main thing that interests me in 4E is a wholescale removal of the major design problems of 3.5E. These include the basic game-breakers, usually involving power imbalance and rules quagmire. While in multiple dozen splatbooks it must be impossible to avoid the occasional unbalanced item slipping through, I wonder if the core books have been tested hard by optimisers.

There are many well-documented things that need fixing that I assume you have under control (polymorph, antimagic, persistent spell, various high level spells, druids, clerics, etc.)

I think the thing that gets the least attention is the timing system. On many occasions my characters have a spare move action and a handful of swift actions that they want to do. House ruling that shorter actions can fit into larger ones (swiftmovestandard) causes all sorts of balance problems. (e.g. that would allow 3 spells per round) Are there any plans to revamp this?