Jumping puzzles. Basically hidden puzzles around the world in which you have to do some jumping, maybe solve some puzzles on the way and then you get a treasure at the end of this jumping puzzle.
The jumping in itself can sometimes also be a puzzle along the lines of: "Wait, how do I get up there? To which block do I have to jump now, to reach that."
The cool thing with these would be, that they can be very different in terms of gameplay and complexity.
These jumping puzzles could be more jumping and less puzzles - or more puzzles and less jumping - or whatever, there's no strict rule set for these. They are basically just hidden minigames around the world.
And they can be hidden in a way, so it doesn't take away from the actual experience when walking around the plaza.
For example, you could hide a small button in one of the water pools in the plaza, which then opens a secret door in some wall for a short amount of time - If you run there in time, you get to play a secret jumping puzzle.
Or let's say there's a button on a fountain in the middle of the plaza, which causes the fountain to shoot water. The water shoots you up onto a cloud and then up there you have to jump from cloud to cloud - Stuff like that, y'know.
I would really like to see something like this in Tower Unite, since it is also a way to add some more secrets and depth to the plaza - And the developers would have a lot of freedom with designing this sort of stuff.
The idea came to me because of Guild Wars 2 - They have hidden jumping puzzles throughout the whole map and it is a really nice addition in my opinion.
Here's a little example: https://www.youtube.com/watch?v=maKi8YdDatg&t=50s