You are probably right that there is eventually a point where your skill at something like toribash is hard to improve (since it is to do with knowledge of how the game works and you can eventually know practically everything to do with the game). However, there are practically unlimited mods to master, once someone is as good as they are going to get at parkour and tricking they could change the player-model to something completely different like a wolf and do it with that, although they will still know the physics the new joints will have different capabilities and running, climbing and jumping techniques will change. After that you can learn to use different weapons, staffs are generally viewed as easy to use and as unrealistic but if someone made a well weighted enough staff I can imagine that it would be possible to do some staff twirling and suchlike. There are skateboard mods, skiing mods and biking mods, and there are so many alterations which could be made to these mods that I doubt anyone would have time to master all of them.
As well as this there are also the possibilities to mess with the mechanics, self grabbing is probably glitchy enough to do some pretty unbelievable stuff with if you turn dismemberment off and get your timing right. I haven't seen this sort of glitch utilised before, maybe because it is impossible to or maybe because people haven't thought of how to yet.