I think they could do a lot with the Towers to extend the life of this game. Its something they haven't done with previous games, but could start with this one.
1) Titan Shang Tsung should be more of a Titan. Not saying he should be giant size, but there's no way he should play the same as the playable Shang Tsung. Buff him and give him back his classic abilities in addition to the ones he had from MK11.
2) The boss fight at the end of the towers should alternate between Titan Shang Tsung and Quan Chi.
3) Secret fights. We got no new characters in this game, but secret fights is how we were introduced to new characters in the classic games. Those characters later became classic characters. You could even give us a fight with Chameleon allowing him to be unlocked under certain conditions.
3) Sub Boss. Ermac felt like a sub boss in this game, but it was missed opportunity to have him in the towers. I know he's DLC, but we get to fight against him in Story mode, so there's that. I'm sure they could put an amplified version in the towers. That or they could make Titan Havik the sub boss.
4) Mirror Matches. They could really get creative with this. Your character faces off against the version of themselves from another timeline. This also provides an opportunity to incorporate classic pallettes/skins that could be unlocked by winning the mirror match or a completely new skin.