Do you genuinely mean that it's opening next year, or do you mean that they're submitting planning permission or building it next year? The latter option seems more plausible, as I'd imagine that planning would have already been granted by now and Alton would have been hyping it up far more if it was opening in 2020, not forgetting that Wicker Man only opened last year, but stranger things have definitely happened in the theme park industry than SW9 suddenly being announced for 2020. It would certainly make 2020 a very exciting year for both Towers and the UK theme park industry as a whole, though!
However, I could potentially see a 2021 opening for SW9 if Alton files for planning permission fairly soon. 2021 would represent 3 years since Wicker Man was built; this may seem like a short amount of time between major investments, but this is the same amount of time that came between Thirteen and The Smiler, so I certainly wouldn't rule it out. It is also worth remembering that going by Merlin's 4-year CAPEX cycle (peak, low, medium, low), Wicker Man actually came a year late (it was supposed to come in 2017 going by this cycle), so perhaps Merlin is giving Alton SW9 a year early in order to balance things out. I'd also say that Alton Towers is a park that Merlin can justify major investment into; all 3 of Merlin's major investments into the park have been extremely successful.