From my experience of building water rides (roller coaster tycoon 1,2, & 3 + expansion packs). I think that there is enough room to place a “big drop” ride somewhere inside of GRR. All the machinery is there to pump the water up and the mountain to hide the track.
Really, if you're going to put another ride in Grizzly Mountain, I'd rather it be a mine train ride a la Mine Train Through Nature's Wonderland or the Calico Mine Ride at Knotts. If you're going to put a big drop ride into the park, the best location would either be where the Maliboomer was originally located, utilizing Screamin to hide some of the supports and drop it into the bay, or building it into Pacific Wharf, taking out the Bakery Tour and utilizing that for a queue/loading area/lift hill, then dropping the ride into the water area right there.
GRR is enough for me also. I don't need to feel like I just jumped into a swimming pool. :)
Adding one more slow water ride seems to be what everyone wants.