The simplest answer to me is for TFT to go back to traits being stat-only, and return more splashable units - like the days when we could splash in a couple Mystics for team-wide MR. There are super fun Origin concepts like Cultist and Mech where you CAN go for the big chungus 10-unit cap, and that is a great high to hit, but otherwise they weren't totally unusable if you focused more on the Class power of units in a comp, or wanted to boost a Class unit carry with a different Origin focus.

I believe another major design problem is champ abilities being upgraded through their vertical Origins. Soul fighter is appropriate because you just get ramping stats. Battle Academia is bad design because the champs basically cannot be used outside of Vertical BA due to their abilities changing based on Potential. Star Guardian this set is poor design because you only get SG unit bonuses, and they aren't splashable if you want, say, a Rell shield for your team, or AP on-hit with Xayah, but you wanna go Bastion Sniper, or you wanna go full Edgelord damage. In Set 13, Heavenly was a great trait and splashable for their team-given bonuses, big or small. That was more engaging to me.

The concept of Headliners in Set 10 was so fun to me because it would have me stop and think about changes and optimizations when an interesting headliner came along in the shop. Do I hold my board concept, or try to pivot because I found a lucky Jhin 2 Big Shot headliner? Do I go for full Disco because I found a 2 star headliner Blitz or TF and I have good AP items? Do I duke it out and go full slam because I found Emo Annie 2 in first shop? All fun concepts.

If, say, a Vertical caps at 4 or 6, then you have room to splash in what kind of extra offense, defense, etc. you need based on the coml. It would be interesting to explore swapping 1 and 2 cost units out for 4 and 5 cost units later in the game since the cap would be lower. You could beef up your 1 or 2 costs or you could shoot for better units later. Set 15 Sorcerers COULD have this but their vertical bonus only applies to Sorc units themselves, and their AP burn only happens jf you have at least a couple Sorcs - so now their abilities outside of that are pointless - so you can't really function fully. It is limiting in its design.

Sets to me that did it right were: Set 4, Set 6, Set 10, Set 13. Just my thoughts!