I posted something about this on our site, but I think part of the reason that there's so much dead space in Epcot is because the park was overbuilt in 1982. Expansion to the park wasn't necessary, but the park still needed to evolve. This meant things would either be replaced or closed.
Capacity has never been an issue in that park because when it was first opened every attraction had a ridiculously high capacity. That's not to say that high demand/gate busting attractions couldn't boost the attendance for that park, but I really don't see them re-opening Wonders of Life with a new theme. I would expect a full demo/rebuild of both Wonders of Life and Universe of Energy.
It's entirely possible to have a total wait time of under an hour for every attraction in Epcot except Test Track and Soarin' on any given day. This is a good thing for those people that like to park hop, but it doesn't really help drive attendance.