Kits to build slides would be very cool. It would be nice if we could build a platform or landings that can have slides and let the landing be shared on a single tile with stairs and slides. It would be extra cool if we could have different styles of slides similar to how we have different styles of cabinets.
Waterfalls that can be interacted with would be cool too. There could be a piece sticking above-- maybe even a rock formation-- with water coming down that Sims could splash in. Perhaps the steps for the stairs could stretch over a portion of the pool and there could be a little cave underneath where Sims can swim.
A diving board would be nice.
I will add that stairs inside pools to get out would be nice. Even if they don't go all the way to the bottom. I wish I could get better satellite images of my old house, but I had a pool that was sort of shaped like a peanut that had stairs on the shallow end but on the opposite side in the deep end there was a sort of step that worked as a seat/standing area. We were able to get out of the pool without using pool ladders but we did have a diving board.
We had some sort of powered thing on a hose or something that would float on the surface and sweep up leaves and bugs. I spent a LOT of time in my pool as a kid. After my homework was done, I loved jumping in the pool and swimming around. Even just floating in the pool was relaxing.