Give rewards from TFT to Twisted Treeline
The only reason why i play this bull is to get the rewards and because having good planned out missions even a not so good game can feel rewarding.
I play twisted treeline much more rarely then summoners rift. But i would not play TFT at all if there were no missions and rewards.
Give twisted treeline the same treatment as you give TFT (weekly missions and rewards) and you'll see their numbers rising drastically. You can even reexamen the map and freshen it up a bit. Limit the champions that work in twisted treeline and start from there.