My mind immediately jumps to thinking about branching valid time timelines, but having thought about this a _lot_ over the last few months I think actually all other kinds of timelines can be implemented using valid time + additional indexing.
I thought about it, too, but I guess then you really dive into the field of versioning systems and probably it won't be a database system anymore. That is also manual conflict handling...
I think branching/merging itself wouldn't be that hard to implement, at least if you have a versioned index at the very core (disclaimer: I'm also developing an Open Source temporal storage system). But then you'd have checkouts, handling conflicts...
