Haha I've had this thought before. A MLP amusement park would be pretty amazing. The rides would be sweet, the prizes would be awesome (I'd play an amusement park game relentlessly to win a MLP plushie), and it would be a great chance to meet other bronies. Sadly, it's pretty unrealistic. Disney is HUGE, and even the biggest Disney titles only get a small section of the overall park. A stand-alone MLP amusement park would not be able to make much money, since an amusement park needs a lot of visitors. Although there are a lot of bronies out there, we're not nearly as big as Disney when it comes to fans. Disneyland and Disney World are pretty much the places to go to take your kids, so a lot of the visitors of Disneyland/Disney World aren't even that big of Disney fans. It's also a popular place to take your significant other, since Disney (like MLP) is something people of all ages can enjoy. Although MLP shares those same qualities that can rake in visitors and gain popularity, it's not very popular compared to Disney, and it's a stand-alone title, whereas Disney is composed of a bunch of huge titles.
The most realistic thing I can see is My Little Pony having a section in an amusement park, with few rides and some entertainment sections. Even though MLP isn't Disney, maybe they could one day put in a MLP section. Given the rising popularity of MLP, I wouldn't be totally surprised if Disneyland/Disney World or another amusement park did this. Super Smash Bros. started adding non-Nintendo characters, so this kind of thing wouldn't be completely unprecedented. Perhaps one day we'll get a little section, but I highly doubt that we'll get a whole amusement park dedicated to MLP. It would be amazing for MLP to have its own amusement park, but hey, even a MLP section would be pretty damn awesome.