Wow! That's tough! I'm am leaning a bit towards the Magic of Flight theme as it would mean the possible return of the classic Balloon Ride.
However, I don't how that would fit into the playground itself. Especially since theoretically, Silver Dollar City is set in the 1880's, and the first airplane flight wasn't until 1903. Yes, I know I am looking into it way too much.
The Fire Station idea is neat, but it seems out of place with Fire in the Hole being across the park. However, it would work out well for the existing water cannons.
The Fireworks factory would be a hard one to pull off. I think they should go with the logical approach, and turn it into a riverside dock play area. This is already hinted at a bit currently, however the entire structure is modeled towards a City Hall which is very out of place.