Now that we know it is going to be a new building, we need to try and make sure it is a great one. It wont be a Peckham Library or a Sydney Opera House - we need to be realistic about the money available - but there is no reason why it shouldnt be a fantastic local facility. Ultra modern does not have to mean ultra high-tech (with all those gadgets that immediately go wrong), I would love to see a basic, functional, contemporary and ultra eco-friendly building.
How it is used is as important as what it looks like, and the council (and local residents) should be planning now about what services and facilities will be there.
With effort, creativity and imagination I would think that most of the many uses people have suggested on this thread could be acommadated. Entrance foyer that doubles as meeting hall, smaller fitness rooms that double as meeting rooms, even a cafe that becomes a restaurant in the evenings!

By the way, i used to use the pools regularly as a kid. all the lockers had a number and you had to use the locker that the life-guard told you to. They were in blocks (1-50, or 51-100 etc). Every half hour or so the life-guard would blow his whistle and hold up a board with numbers on it. If your locker number was on that board, you had to get out! The best game was to try to sneak an extra 5 minutes in the water. hey ho, happy days...