Riot create a new live branch of summoner's rift

Try to name it something that doesn't preference one over the other. E.g. No Summoner's Rift 2, Summoner's Rift Alternative, exc...

And upon release, just make it a clone of the current game. Hire on new staff to develop for the new game mode, and have them start developing the game, at least initially, in the opposite/tangential direction of the current development team.

I'd even start splitting the champion designs across game modes. Following the same themes, but instead of reworking a champion like Akali to be more... well however you want to describe her current state... they take a champion who's in a state similar to that of your target, and rework them into a state more akin to that of the champion you're reworking.

Anti-Rift!... naw, that also sounds like a subsidiary. Just call it like Champion's Divide or something.

Really, what matters is that your community is divided among multiple lines and you're trying to stretch a game with limited flexibility into appealing to too many people. If you just make a new branch of the software that can develop in a different environment to appeal to a different audience, then when you see a major shift in the market or the environment change, then the more variations of your game that exists, the more likely one of them will survive and continue to grow.

I assume that some of you understand the concept of evolution and speciation. If you only have one version of your software that's live at any given point, then you're praying that mechanisms beyond your control in the environment (gaming market) don't change unexpectedly or too abruptly for you to change course. Or that nothing new which dramatically changes the market dynamics appears... like a new genre

And yes, this would include non-game related features being duplicated, like sponsored tournaments and a ranked mode, but make them to scale.