I would hope "never" would be the answer to this question.
I have no idea if I am in the minority or the majority but I think Disney needs to keep one thing in mind with all changes. To me at least Disney is a destination above the other parks in central FL not because of the "rides" but because of the "connection" people feel to their brand. I think people feel a connection to times past to their childhood or to their children's childhood. Be that from a movie or a ride. I am not saying Disney should just let their attractions get stale and decay, upgrades updates and additions are good but what I am saying is that they best not lose sight of what makes them "different" from say Universal. Play to their own strengths do not try to emulate others.
I have read on here that HS when it was originally planned was sort of as a response to Universal and that it ended up changing soon as it was not super successful. Again stick to your strengths Disney. Universal is great as a place to go if you want to be thrilled and entertained but at least for me there is not that "connection". Disney as i said before at least for me connects to something more and they need to be careful that they do not sacrifice that connection part while trying to become more thrilling or entertaining.
On the original subject of TGMR i think if done right a Mickey ride could fit what i am saying here. But it needs to be done in the right way and not just some technically advance thrill that does not truly connect the rider to Mickey's story and maybe even go behind that to incorporate some of what went into Walt coming up with him.
I also agree with those that say that all the parks should have an over arching theme to them. If i had my way I would center HS around Disney,(Pixar, Lucas etc.) movies and all the Disney characters. I would have MK sort of the way it is as the "fantasy" type of park. The other two parks I would have a learning centered approach to. EPCOT I would go back to its roots and have it be a true epicenter of learning about the world around us from different cultures in world showcase to science and technology in future world. AK i would focus on the aspect of all the diversity of life on this planet (kind of already fits that).
Sorry for the long post, just had a few minutes to spare and thought I would spill my thoughts all over the page. lol.
Ok, I now respectfully step down from my soap box.