(I know Galaxy’s Edge isn’t technically this decade but it’s close enough to be relevant to the discussion I think. It certainly goes to show that we can receive something Earth-shattering right at the end of the decade…)
According to a fast Google search just now, the first legitimate rumors from reputable online sources about a "Star Wars Land" coming to Disneyland were from late 2013. The land opened in two phases, six years later in 2019 and 2020.
So... does that mean if we start getting some solid rumors about something new being built in Disneyland later this decade, it would open in 2029 or 2030? History seems to suggest so, at least for major projects.
Cars Land was publicly announced by Bob Iger in October, 2007 and opened in June, 2012. Five years from official announcement to opening. A massive remake of DCA was being rumored online by reputable sources by the end of the 50th Anniversary in 2006. Six years from strong online rumor to opening.
Disney's Fiscal 2024 starts in two weeks. If bulldozers arrive in Anaheim in October, that would be a good sign they might have something new in either park by 2027 at the earliest. But history seems to point more realistically at 2029 or 2030.