Rotating Game Modes, they should be more.. fun
Poro King is fun and all but other than the giant Poro King in the sky it doesn't really bring anything to the table in terms of constant replayability
Same with Ascension playing it once has quenched my intrigue of it, and the many friends I talked to found that playing it once was good enough.
One For All similar aspect, playing it once or twice was fun enough to make you enjoy it and the novelty wears off quite fast.
For me what these game modes all share in terms of loss of enjoyment(I'm sure not everyone on this thread will agree with me) is the fact that it doesn't really have anything "fun" with Status and champions. It's basically just normal mode with something slightly different about it. Not much.
URF gives you 100% CDR, it gives you infinite Mana, it gives you fast gold regeneration and loads of other fun little status things which does keep interest in it for quite some time, at least until insta lock Soraka and healers becomes a thing and people maintain a fun pace and explore different characters.
I think if Poro king had something interesting to it in terms of status for the Champions, like Infinite Mana for the players with the Poro king, or something like Increased Attack Damage, Ability Power, and Health/Mana regen would also be kinda cool cause it would make the Poro King something heavily contested even more so.
In One-For-All it should just be URF but with the ability to use 1 champion so I can have Hook City come to life!
Game Modes should all have something interesting with it, instead of just being URF that gives you all these great benefits of playing it, which always makes it heavily anticipated every year it's April Fools, Poro king should give a team buff or just faster FASTER Gold generation, Ascension should do something(Haven't played it for a LONG time kinda forget what it's about) One-For-All should be URFOFA, 6 v 6 should have 2 Blue Golems, 2 Red Golems standing side-by-side so 2 junglers can be a thing(Or not.) with 5x the gold generation and whatnot..
I dunno I'm just rambling on at this point repeating myself.
I just think Game Modes should have funny little quirks with Status' like URF has. Rather than have 1 thing it shows off and just kinda remains the same old SR gameplay.