It's too obvious and makes too make sense to do that, so they rotate game modes.
-URF should have been enabled in custom 4 years ago.
-All for One would be one of the most useful mode for teaching a group of 9 people 1 champion.
-The 1v1 mode should have been added on it's release.
-Nemesis draft has potential for it's own meta, and would always be fresh with the shifting meta.
-Doom Bots is fun to test your ability to carry, and is even decent practice and more refreshing than intermediate bots that are indistinguishable from beginner bots in their decision making.
-Hexakill is fun, unless you only have 4 friends and have never experienced having 1 or 2 too many people in your gaming communities. There's always that guy that's too nice and spectates when he could be playing.
-Ascension is fun now that it's fixed, and is just as refreshing as 3v3 map for variety factor while it's around.
-Bloodmoon and Overcharge give ADC and Assassins decent scrimmages at faster paced gameplay.
-Who didn't like Invasion? It pisses me off I can't access that anymore whenever I want or that it might be gone forever. Just like the rest.