Currently there is no construction going on at the lagoon athough I've hear many rumors that they will build something there. Personally I think it will be awhile before they make that area into a new ride. If they use the existing lagoon to create a new ride they have a problem. No place for a line. What used to be the submarine line is now the exit for the new autopia ride. A shop decicated to the ride where you can get a picture taken inside one of the cars, plus by the chevron cars, and various other suff. They have also used the area as the line for the monorail. This make's it nicer since you no longer have to stand in the sun, you are now in the shade.
If they don't use the lagoon then they have the problem of having to take it out, so that would take awhile also (and I doubt they would go to the trouble of removing the lagoon, but you never know).
Just my .02 worth.