The problem about replacing Jordan with anyone is that no one is Jordan. We'd have to think outside of the box for someone who could play it technically, or at least 90% of it, all but was a completely different kind of creative genius that still operates in the Dream Theater genre and I don't think I know such a keyboardist. The reason why I agree with people who suggested Derek is because a) he has advanced severely since leaving DT, and he was pretty adept to start with, I don't care what anyone says b) he's made some very good music with DT before.
That's all there is necessary, to be a good musician, technically capable (though I wouldn't mind DT going for less technical stuff in the future) and very creative and to mesh well with the rest of DT. I wouldn't expect of a replacement singer to have JLB's timbre and uniqueness and I wouldn't expect of a replacement keyboardist to work on new technologies like JR. We didn't expect of MM to oversee bootlegs, DVDs, be the public face of the band, sing backing vocals or write lyrics either.