Bellow are some ideas that i think will help improve tft.
_**Content and cosmetics**_
1) Gradually add all the champions. Obviously this will take time but i am stating it in case riot doesnt plan on adding other champions.
2) I noticed that all champions are on their base skin except vayne who is on archlight. That gave me the following idea. Let's say i own queen ashe. You should let me pick which skin will apear on the champions on board, the base or the queen ashe. That way you can use both your skin collection in the summoner's rift and tft.
3) Gradually add more items.
1) You should let more comeback mechanics. At this point, if anyone gets a level 2 draven with 2 items and a kinda ok frontline he will win 70% of the time. If his frontline is good and draven gets 3 items it will be gg 99% of the time. You should let other people have a chance against him.
2) Make synergies matter more than items. I mean you should be more rewarded if you manage to gather 6 pieces for a synergy than gathering 2 for and item.
3) You should make the game pace slower. Increase level cap to 10. Make players take less damage. I mean we already have enough fast paced games on SR.
_**Gold and rng system**_
If you plan on making it a ranked game mode you should reduce the rng mechanic and make it more strategy like. Also, THE REROLLS ARE TERRIBLE.
1) I see people on the late game being sort of 1 garen, rerolling like crazy spending 50-60 gold and not getting him. I mean basically if you reach late game you can never get on rolls early game units. You should make them purely rng and not weighted rng like now if you want to keep that reroll system. Kindred should have the same chance of appearing with ashe, lulu and graves.
2) An alternative rerol system could be the following. Rerol based on how much gold you want to spend. 2 gold for basic units, 3 for rare, 4 for epic, 5 for mythic and 6 for legendary. And based on how much gold you spend you get units only from that tier on your reroll.
3) Stop the buy sell abuse. You can buy any unit you want and then sell it immediately getting the same gold value back. That can get abused in order to build any comp that you like. You should give back on less gold for every unit sold. Get 3 gold instead of 4 if you sell gnar. Obviously a low bound should exist for the tier 1 units, 1 gold minimum sell value.
_**Things i find that are ok and dont need a change.**_
1) The idea that higher tier units are stronger than lower tier ones i think is great. I mean a draven shouldnt be as useful as a tristana.
2) If you forget to add a unit even though you havent reached the unit cap you instantly get one added. That is awesome.
These are the things that come to my mind right now. Might edit if i remember something else. If you agree upvote so that riot devs can actually notice.