I think it was a post that mention that Ice Age was being developed by Pixar and that is just not the case. Actually, Ice Age's second trailer seems to indicate that it has a little bit of similarity to Monsters, Inc. Crazy looking creatures have to get human back where they belong.
As for rides and other stuff, I think it will hold its own over time. Toy Story has been an interesting presence in the theme parks with a cool ride at Tomorrowland and character presences but nothing in the E-Ride class. A tour through Monstropolis with wild consequences could be an interesting ride. I would agree that John Lassiter will have input on any future attraction, especially with his prior involvement on the Space Ranger Spin and It's Tough to be a Bug.
The impending battle between Monsters, Inc. and Harry Potter is not much of a contest because Harry Potter had Star Wars type hype attached to it. The real battle is coming between Shrek and Monsters, Inc. for the Best Animated Film award at the Oscars. It seems as though a lot of critics think Shrek will win and that it deserves to win. I seem to disagree on both counts. Dreamworks has a very powerful lobby at the Oscars; they have won the last two Best Picture Oscars (and they should have won for Saving Private Ryan). However, the issue that I feel may sway things could be who exactly gets honored for Best Animated Film. Is it the director, producers, or both? If it is the director, it may be an uphill climb. However, if it is the producer, etc. or both, I think there may be a campaign for sentimentality to win out and honor Pixar for the three films that came prior to Monsters, Inc. that didn't get such an honor. They come up with this category this year and not in years when it would have clearly gone Pixar's way.
If JyI was going to be rethemed to Monsters, Inc., I've said it previously that it would be cool to theme it as a tour of the factory with the doors representing doorways to the imagination.