The big issue with having any real amusement park created by Frontier to place in the game is licensing.
Disney likely won't license their rides/characters to Frontier, they'd enlist their own studio and build their own "Disney Theme Park" creation game, without stuff that is non-disney appearing in it. I'd love to see this happen- but I don't think Disney is even considering it right now.
Six Flags only has Amusement park rights to the DC and WB characters. This means the only assets they could license are the Six Flags brands, which I'd love to have access too in the game, but honestly aren't nearly as detailed as what Frontier has come up with already.
Universal wouldn't be able to do license the Harry Potter stuff, but they could do any Universal stuff (unless they've licensed out the game rights to any specific IP).
Basically, all we'd be able to get are pieces of real amusement parks, and likely at great cost to Frontier, so they probably won't bother. Once UGC is implemented though, I'm guessing we'll see tons of Disney, Six Flags, and Universal content uploaded and full, accurate recreations of amusement parks created in PC, assuming Frontier turns a blind eye to the copyright violations.