Team up abilities are such a pain to equip every match and it is one of a huge reason why TU feature are so unpopular/dislike.
I think that a very simple feature will make team up ability a lot more relevant and comparable to what environment use to be without being grossly imbalance.
If you enter a game with empty boost slot/s, the game should randomly choose Team Up ability from your available pool and fill up those slots. So for example if you enter the battle with AP boost in 2 of the slot and one empty slot the game should assign a random TU ability to the last slot. If you enter the game with 3 empty slots then you get 3 random abilities. Team Up will still be use up as it would when you choose them manually. If you can randomly give Team UP to AI i am sure you can do this.