I think the future DLCs would benefit greatly to replace themed DLCs with packs that contain more gameplay features such as rides and features.
A coaster pack which consisted of 8 or 9 coasters, but it also featured new coaster elements such as on-ride photos that visitors could purchase in an all new exit-queue vendor that sold on-ride photos. The coaster pack also included some new queue elements such as digital wait time billboard, mist fans, and queue canopies.
And much like you posted with a water attractions DLC, it would be a great DLC for a summer release. New rides included: Super Flume (Splash Mountain boats), Shoot the Chute, River Battle (interactive boat ride with water cannons- perhaps a Pirate Ship theme) and a boat ride drawing inspiration from the Pirates of the Caribbean ride). Some additional features included in a free update- colorable water, pathway geysers and water cannons, and new water shop display feature with the ability to create synchronized water shows.
A Dark Ride DLC would be one of my top choices which came with 7 to 8 new dark ride types (free update featured an Omnimover aka Doombuggy). The Dark Ride DLC also allowed us to create motion theaters using digital billboards to sync with the motion of the theater seats. Other ride types- Rail Suspension (inspired vehicle from Peter Pan's Flight), Blaster (riders can shoot at objects as they move by), train (inspirations from Casey Junior Circus Train). Also, with the new Dark Ride DLC came with a feature that allowed dark rides to remain dark no matter what time of the day is. A new paint set could also be released where we could create fluorescent/black light effects on objects and props.