A Re-theme or re-paint of the Neptune's beach area would make sense and wouldn't require planning permission. Due to the planning permission for the dark ride in 2016 I doubt anything will be done withe the Fungle Safari site as they would have submitted a planning application at the same time as that.
The generic attractions currently missing from Thorpe, when looking at other Merlin parks are
-family dark ride (ie: Laser Raiders - Legoland, Duel - Alton towers, Tomb blaster - CWOA) due to arrive in 2016
-Sea Life center (ie: sharkbait reef - Alton towers, Chessington Sealife center - CWOA, Atlantis Sub-marine Voyage - Legoland) no planned arrival as of yet
Based on that it could also be some sort of sealife center although I would have assumed they would have needed planning permission for it.