It would be cool if we could build pools with a type of puzzle piece system similar to the foundations. One type of puzzle piece could be a sloped sort of "beach style" entry. One piece could have benches around the edge that sims could sit on. Another piece could have various options for waterfall features. Also, we should be able to have curved edges for pools. I don't know why we can't.
I agree that slides and waterfall features need to be a thing! Also, remember the pool nectar bars that we had in TS3?! Those were awesome! All in all, pools need some updating. Could be a great kit or stuff pack maybe.