Plans for a 4th animal attraction (in addition to Singapore Zoo, Night Safari and Jurong BirdPark which are managed by the same organization) have been green-lit from what I've heard.
This proposed "River Safari" will focus on freshwater ecosystems around the world. It will straddle across 2 adjacent plots of land from the existing Zoo and Night Safari (blue line on map) and incorporate the Reservoir, with ferries as a means of transport.
More info on the Singapore Zoo thread.
*Note: Ignore the pink line. This plan is a bit old so some of the listed exhibits are incorrect. Thanks.