I've had this thought before but theres a few technical limitations on this and other custom things you see in things like OMs, namely the fact that these changes are isolated into "mods" within the code which isolates the changes from the base code for the current generation. In fact, all past generations are mods themselves as they change the rules for the current generation. At this time the only way to set the mod a format uses is through editing the code itself, its not supported via custom rules as that could break things.
Speaking of breaking things the other big concern is that I'm worried the custom moves/abilities/items for SSB will break if used outside of the intended context, possibly crashing the server. Some of these moves do things that the engine really doesn't support so the code can be hacky and brittle in some cases.