..if it will happen at all someday
I just can't see why it's not possible to create some kind of api-based scriptable engine League Commissioners could modify, adding/rewriting skills and creating new mechanics. Something close to what Arma project does, if you are familiar with it a bit. That would be absolutely innovative and fascinating feature bringing game a lot of love from the core community, and allowing it to implement their own crazy ideas in a massive MP environment.
Though BB's mechanic as a whole is rather complex, its separate elements are not that much. You can relocate ball around the pitch, attach it to different players, apply some additional transmutations to its current location. From this point of view, say, Pass action could be perceived as calling a Pass() function, which first would check whether the object it was called for is allowed to pass (he may have No Hands, or be under some special effect preventing him from using pass), then it would check coordinates to which ball is relocated, using a range table as a reference to devise basic roll that must be passed; then it would call another function checking what possible modifiers could be applied to it. Modifiers_applied() function would check what skill affecting this action the passing players has, as well as how much tackle zones he stands into. Etc
I.e. it's clearly could be scripted in some very easy-to-read form, using Python for example, and then allowed to be changed by members of community ready to delve into this. Thus allowing them to rewrite ANY aspect of the game literally. Or at least anything for what api is provided at this moment (then Cyanide could expand this api, on demand). After all changes are done, it then would need to be uploaded to some custom league by Commissioner with enough access rights, and set as the main ruleset in there. Then it would be signed with some secure private key, allowing all game clients of this league's members to download it and make sure it's indeed a file distributed from Cyanide's servers, and it was not tampered. To prevent your opponent from tampering with the ruleset scripts on his side, your own client would verify every his action and roll against your own copy of ruleset, quickly detecting any discrepancies.
This core model could be then used in future games, as it's fairly "high-level" and platform-independent. Any issues due to broken mechanics in private leagues due to such customizations could be not covered by Support and would be left to discretion of its players and Commissioners, thus it won't be issue for Cyanide.