The things we have to consider when looking at potential changes are: 1) Price, 2) Space, and 3) Ease of Implementation. With that said, I think in the event of a third attraction being added (he says with absolutely no knowledge, just speculation) I think that it would be the Parachute Drop.
Here's why: they'll want to bring a clone in to keep costs down. RC Racer (coaster) and Slinky Dog Spin (same theme) are redundant, so this is the only Toy Story attraction left that already exists elsewhere and would add something to the area. It also takes up a (relatively small) footprint, and would partially make up for a loss of theming that could come from further dumbing down of the coaster.
I can't think of what else they could do that would solve the problems they have (too much money for too little capacity), and if I'm not mistaken, Parachute Drop is the most popular attraction within the Playlands.
The only problem I can think of is that of sight lines, particularly those coming from Star Wars Land. Then again, there are ways to get around that (elevation changes, strategic facade work, etc.), and placing the attraction on the right side of the land would help, provided it doesn't interfere with GMR sight lines.