Implementing this later on would probably be very messy, I agree. If you look at Minecraft for example you can see it is a feature people crave for. TM also shares the same simplistic approach in the editor as that game....so it makes quite a nice analogy. Minecraft was created by one single guy which makes me more confident that the Nadeo crew had time to work on this, besides working on three game titles with an amazingly small crew ^^
Network capacity would not be an issue I think, depends on how much freedom you have in the new editor. But if this feature were implemented in TMU I don't think it would require much more traffic than that of a multiplayer replay...which would make this work with dozens of builders
