Knowing BAA, they'll probably build an indoor observation deck looking out over the huge Terminal 5 shopping area. There will be maps and guides explaining each shop, what it sells, and where to find other locations at Heathrow (the Sock Shop map will cover a whole wall.) There will even be a live feed on a loudspeaker, so you can listen to the employees at Costa Coffee taking drink orders.
I think people here are getting far too excited about Terminal 5. It looks like a beautiful building with lots of glass and natural light, but remember that we're dealing with BAA here. They'll find a way to muck it up, and at the end of the day it will just be another shopping center with HSBC-logo jetways attached to it.