Featured Game Modes are Riot's attempt at bringing "something fresh" to the game. They rotate game modes because they need to rework/rebalance the game mode once they have released it (hence, why they released One-For-All a second time, same with Hexakill).
And they rotate game modes also because they don't want to stay with one thing for too long, so they are constantly trying to make 'innovative' games to try and make League games more interesting. Riot probably has a very special team that is solely dedicated in game mode development, and they probably test the ideas that they make.
The only exception to any game modes that they have ever released is URF mode because everyone liked it. It was very fast-paced and full of action in such a short time. It's the most stimulating game that they have released (which is why I and MANY MANY other League players are hoping for a permanent return of URF).
When they re-released One-For-All (Mirror Mode on Howling Abyss), they made it so that both teams played the exact same champs. For the second release of Hexakill, it was played on Twisted Treeline instead of Summoner's Rift.
Personally for me, for both of those Game Modes, the first releases were more fun...
I really hope when they re-release URF mode they'll keep the same exact "Awesome Buff". I also hope they nerf/balance some specific champions just for URF mode (like Sona, Nidalee, Hec, etc.)
But even if they auto-ban/remove champions this time around, I'm still okay with that....as long as I get URF mode back.
I HOPE THEY DO NOT DECREASE THE CDR FROM 80% TO SOMETHING LOWER!!!!! Keep the Awesome buff the same ploxxxxx RITO!!!