1. Similar things where already proposed
Especially inter-world or even inter-region GBG was already ruled out by Inno due to technical difficulties with keeping all the servers in sync.
2. You want to create a whole new feature for only the top 4 guilds? What a waste of time...
3. The top 4 are already playing in a league of their own, as technically every battleground is its own league. The copper - diamond "leagues" only are prize categories...