The intersection of git and wikis is interesting, it's one of those things that sounds compelling but then the details usually end up making it hugely complicated. Worth thinking about.

For general writing collaboration I would seriously consider not allowing arbitrary branches. Most people just get confused managing multiple independent outstanding branches that potentially branched from different points in the history.

One restriction could be that each user gets a single branch (their suggested change). As changes get merged into HEAD each user's branch must be rebased (with a click) before they can add more changes to it. From a programming point of view this is restrictive; why can't I suggest two independent features? But keeping track of dependencies is precisely what's hard for people.