I know this is a big reason why it isn't done anywhere else...but Disney has plenty of room to work with, and in the case of Volcano Bay, these slides are lower to the ground and more terrain-following. They could build slide complexes around it and on top of it in some cases....
That being said, I'd much rather see stuff like this show up in a whole new waterpark on the south universal property...take the waterpark in an entirely different direction from VB.