You guys are thinking too big, too much. Gree can't seem to even update content or add heros without majors glitches. You wanna try to merge 5-6 servers into one? And most of you are forgetting about s1, leaving them to rot. Let's not forget s1 is the original beta server & s2 being the only other to ever beta test. They are the oldest players in the game. S1 needs a merge as bad if not more than any server. But there's a problem, as mentioned runes are quite different. There is no way to merge s1 before releasing runes 2.0 which as far as we know hasn't been a thing at all since Tactics left, got fired whatever.
The best way to pull off a merge given grees track record with bugs is to start off small. Merge s1 & 2, s3 & 4, s5 & 6, s7 & 8, s9 & 10. But before that can be done they need to work out the current bugs & new runes need to be released. Without new runes s2 players are the only players that even stand a chance at being competitive against s1. No other server has the power to even compare overall. But then s2 players would need major rune work to keep up with the players already using 6* +15 runes. It would be a mess. And to do a merge without including s1 wouldn't be fair.
If gree could manage to implement a new rune system & bug fixes for all known bugs to take effect during a server merge & do it that way that would be awesome but I don't see that happening. It would also mean less servers to keep track of, hopefully making it easier to isolate & fix bugs when they happen. But honestly I don't see a merge happening for another year if ever & who knows if this game will even still be alive by then. In any event s1-4 will likely be dead before it does.