Is there some kind of draft to support a chain split+merge, like Version Control systems do? Currently, if I didn't misunderstand, in case of a network split when the two networks merged again only one of them would be "valid" and everything else would be discarded.

This seems overly simplistic and un-technologic for me: a smart system able to handle splits and merge like distributed version control systems do (like hg or git). Of course that wouldn't be easy to do properly, and it's totally understandable it hasn't been done yet, but I would be surprised if nobody even bothered to draft some kind of proposal about it.

Does such a draft exist?
There is one inherent problem with the solution you described in regards to Bitcoin: the block chain is created in a very specific way. Each block in the block chain links to one and only one previous block, and it is encoded in such a way that if you change the internal structure of the block, it is no longer valid. You should look into the block hashing algorithm. Generally, if you wanted to merge two blocks together, you'd have to solve them all over again.

This does not mean, however, that the transactions from the invalid branch are lost. Unless the coins were already spent in the valid branch, one can take those "lost" transactions and include them in the next block to be mined. This would mean that the transactions would end up having a couple less confirmations, but they would still go through.