Change Twisted Treeline instead of deleting it.
First off, I understand that TT doesn't have as many players, but there is a good reason as to why. One reason is that you literally can't get level 6 and level 7 tokens off of Twisted Treeline. Why? Good question. Another reason is that there isn't much of a difference between TT and Summoner's Rift, other than the map layout the champions and items are mostly the same, except for a few.
I would suggest implementing and fixing a few things: First would be implementing new items and bringing back old items in TT, differentiating Twisted Treeline from Summoner's Rift. Bringing back the old jungle items would be very cool. Maybe don't bring back banner of command, thanks. Fixing Vilemaw's respawn timer would also be good, if the enemy team kills Vilemaw and your team doesn't have vision on it the timer appears says Vilemaw is still alive, if your team goes over to where Vilemaw was it changes to "----". I have run into multiple bugs resulting in entire teams or the entire game just disconnecting randomly, one time it was because two champions ulted at the same time. Some champions have an unbelievable advantage in TT and there should be a way to compensate for other champs. Zoe can ult over the wall behind nexus and the enemy team can't do anything. Kayn can escape almost any fight he is losing, Rek'Sai can go over any wall, ganking and the person not even knowing, with Kayn you'd atleast know, Shaco can do the same thing but can not go over every wall. Funneling is a big issue as it's so easy to do, most commonly I've seen Jax Sejuani, there isn't much of a way to counter it.
If there is a reason for people to play Twisted Treeline then they will. There isn't anything to say that TT would be better or more fun than Summoner's Rift for anything. Riot removing their game modes and different maps isn't bringing league more players. The reason we all play League so much is because there isn't many ways to get bored, you can play a different champ, a different role, a different map. Reducing the amount of maps to two would decrease replay-ability. Especially with Riot Games seemingly forgetting about other game modes they have created such as Ascension, One for All, ARURF and Nexus Blitz (which was supposedly permanent, and if it isn't replacing TT then I'm gonna be furious). I just don't like seeing all these things that have been in league so long being removed when they could easily be changed.