The only reason I'd want Shrek to stay for a tad bit is to help with Potter crowds until they can think of a suitable and high quality replacement. The park will absolutely need the extra capacity...very much. I don't think they can afford to have it closed when Potter first opens.
As for a replacement, I'd really like to see a family dark ride. Doesn't have to be huge obviously, but I think a nice little dark ride could absolutely fit in Shrek's footprint if ALL the space is utilized, as well as they can add to the square footage by demolishing a few of the surrounding stores, NY street facade, etc.