This all assumes there will be a "high investment year", considering this terminology can trace it's roots back to the now well defunct LTDP of many years ago.
Also we're yet to see what was meant by naughty Nick redirecting funding from RTP'S to accommodation and where that would leave the parks by 2021-22. We have no idea yet of how they plan to pull this money out. Mixture of operating budgets, and maintenance? Large investments on a 5 year cycle? Cancellation of specific projects that were planned for the immediate future? Money being pulled out of certain parks but being protected in others?
We just don't know how the world will be by then and what the park offering will be like. All we know is that there will be some sort of SW and it will be in the future, hence why this this thread has turned into a bit of a wish list with everyone trying to fill the gaps in between now and 4 years time. Nothing wrong with that of course.
That said, I think it's safe to say that the gaps in the coaster experience at the park by 2022 will be the same as they are as of 2018, no matter how many water or dark rides are added or removed in between. And SW9 WILL be a coaster and it's unlikely any sizable coaster will be built between now and then to change this.
The park lacks long ride time, a decent launch coaster and it lacks air time. I say combine the 3 and design a decent launched air time machine. It would require a massive land mass, probably talking from the drill end of FV to the end of the Air car park. But it could be kept under tree line and wouldn't require anything unrealistic like building in the valley or across the Ropers lawn or anything.