Several years ago Riot introduced a 1v1 duel mode in their All-Star event, taking place on a map called the Magma Chamber. Pro players got to battle each-other in this mode in front of a live audience. After the event, some players were interested in seeing 1v1 or Magma Chamber come to public League of Legends.
This never happened, with Riot giving a few reasons why later on, some of which are alright reasons and some are not. First, "you think you do but you don't" was one example given as Riot cited "the numbers" of community interest weren't really there to make the game-mode public. Riot also theorized that the meta-game of a 1v1 mode would quickly become boring or solved.
Riot came dangerously close to giving the players what they wanted with a temporary game-mode called Snowdown, which featured 2v2 battles on the Howling Abyss and saw the introduction of the Snowball summoner spell, which remained in the regular ARAM game mode. The first iteration of Snowdown featured rules similar to Magma Chamber; killing the tower, 100 minions, or killing the player. This 2v2 dueling iteration of Snowdown has never been seen again since then.
Riot's decision to remove Dominion was very controversial and upset the small but dedicated fan-base. Riot again cited "the numbers" to justify the lack of interest in Dominion. They were upfront with telling everyone they couldn't justify dedicating any development time to improving Dominion, neither could they let it remain in a neglected state.
The concept of Dominion would re-emerge in temporary game-modes called Ascension and Definitely Not Dominion. Again these modes would disappear or come back into the game-mode rotation a few times.
Finally, Twisted Treeline is the last hold-over of the old-days Game Modes, but exists in a state of neglect almost as poor as Dominion was. Summoner's Rift and ARAM receive many patch-updates around every two weeks, while Twisted Treeline may see one major update every year or two.
Among other game modes which have come and gone; Nexus Siege, Nexus Blitz, Black Market Brawlers, and several cooperative PvE modes. Riot has struggled to offer a consistent line-up of ways to play the game. When one goes away, it's either the community's fault or Riot just giving up.
Now we come to the currently on-going 'beta test' of Teamfight Tactics, another "in the works totally going to be a permanent game mode".
It's broken, it's random, it eats your frame-rate for breakfast when trying to load-in to the game, to the point where you miss the first draft. More importantly it's not actually a League game mode, and is a pretty shameless attempt to draw in the interest that surrounds the similar but better products of other companies, such as Magic and Hearthstone. Yes it already has microtransactions, of course it does.
Here Riot more clearly demonstrates their stance on player interest and community; "If some of our players want something in our own game that looks like our own game, we just can't have that. But if there's a fad we can jump on, we're all in." Some people may have been able to tell this was happening during the "beta test" of Nexus Blitz, which featured an in-game event that mirrors Battle Royale games and the ever-shrinking storm of death.
Riot's biggest problem here may be the fact that they unable to expand beyond League. If they're going to do anything it has to be in the context of League, usually directly connected to the client or game somehow. The one exception was that board game they made, but it was also about league and not exactly a real offering for people who like League and are interested in seeing another video game.
Teamfight Tactics could have and should have been a seperate game. Separating the game from League's technical infrastructure would solve any optimization problems. The game could comfortably exist as it's own thing instead of trying to exist in the ecosystem of League game modes with competing interests. More serious development can be made over the long-term by a team fully committed to the game. The participation and reward structure for League players can even be left in tact.
Blizzard Entertainment's product line-up proves this can work, with World of Warcraft coupling rewards with Heartstone and Heroes of the Storm, as well as Overwatch coupling rewards with HotS as well.
Thanks for reading.