I'm not asking for a ground breaking attraction to take place of Shrek. I'd rather have a relatively simple dark ride than yet another 3D show....especially at USH.
Also, to put a ride there, maybe they could demolition the entire Shrek building and fit a new ride in the space and not conform to the old building's dimensions.
Although another cool option, in my opinion, would be to move Special Effects Stage to Shrek so that the Castle Theatre can be made into a totally new attraction, as per the expansion plan. I'd prefer losing Animal Actors and seeing AA enclosed and SES go in there, but another option would be to put SES (or maybe even a Lights Camera Action type show from Singapore in there!)
Regarding Potter, the only Potter related thing I could see replacing Shrek would be a Shrieking Shack attraction, since it's on the outskirts of Hogsmeade, but I doubt that would ever happen.