They already got the functionality to diff between two documents in Word. I use it all the time to see if legal made any changes while "forgetting" track changes.
Maybe libreoffice should work on it for .odt then, together with various VCS plugins (mainly git and maybe hg). It could be an interesting differentiator feature.
Sure, it's hard to diff and merge tree data structures, but it doesn't have to be perfect. Text diffing and merging is already imperfect anyway, yet it's very useful.