Actually, there has never been a ride themed to Mickey Mouse at the Disney parks, at least not for many decades. The new ride in Disney's Hollywood Studios will be the first one in modern times.“There’s a new Mickey Mouse ride”
“Oh, another one?”
'cos that's clearly not worked has it?
But for what it's worth, I'm with Crazycoaster here. While the Harry Potter areas of the other two parks have provided an immense customer draw, they have the advantage of being connected and playing to each other. Guests visit both parks in a single trip, paying for two entrance tickets to experience the entire Harry Potter-themed land. Diagon Alley and Hogwarts are very iconic locations, and travelling between them with the Hogwarts Express becomes a part of the experience. They won't be able to drum up the same excitement for the Ministry of Magic, for instance.
And this new park will be isolated from the other two, without the connecting train between iconic locations. Its eventual Harry Potter area will have to stand on its own, with little synergy to the others. Instead, it will compete for guests' attentions, at least for all the guests who don't have the funds to visit all three parks. A new Harry Potter area at Universal Studios 3 will either be inferior to the USF and IoA lands, and lose customers to them, or superior to them and steal their customers. Universal doesn't need, nor does it benefit from, splitting the customer base like that. Those who come for Potter will go to the best Potter park and do their merch shopping there. At best, the second-best area will get a visit, but won't be quite the same money-making machine.
I believe they won't go for Potter in the third park. They will instead find some other IP as its main draw, something you can't find at the other parks, and which will draw millions of customers in its own right. Like Nintendo, for instance. They could contain the Potter stuff in IoA and USF, possibly even expand their areas to other lands that would instead be moved to US3 (like Jurassic Park or Fear Factor Live) if they need more room. The Potter experience would keep people excited for the two old parks, while the new park does its own exciting things.