I think that the Rapids has a very long future at the park, but I agree that another water ride is not needed, we simply don't have enough warm days.
My 10 year plan would be something like this (tried to keep it realistic):
2018:
Wicker Man
(Cloud Cuckoo Land closes)
2019:
Cloud Cuckoo Land Re-theme
Enchanted Village Barrel accommodation
2020:
CBeebies Land flat ride
(The Blade closes)
2021:
X-Sector flat ride
2022:
SW9 Forbidden Valley RMC launched T-Rex
New Hotel
2023:
Re-themed CCL new family coaster
(Nemesis Sub-Terra closes)
2024:
Nemesis Sub-Terra IP retheme
Waterpark expansion
Splash Landings Hotel expansion
(Mr Bloom's Allotment closes)
(Rita closes)
2025:
CBeebies Land new experience
2026:
SW10 Dark Forest B&M 4D coaster
Car Park expansion to compensate for partial closure of Air car park
(Duel closes)
2027:
Duel retheme & upgrade
Nightlife area (Air car park)
Nightlife area hotel
Monorail overhaul