Seeing both bands playing "Under a Glass Moon" together.
And there might be another song off of the first SymX album that could be done to balance that out...I actually think LaBrie could do a nice job with the vocals. (And I'm not being sarcastic, either.)
I think it would be awesome if Symphony X switched their sets up a bit, like Dream Theater does. They may not have as big of a back catalogue as DT, but it would still be interesting.