The 50th isn't some giant milestone Disney can just go to sleep after. Universal is going to be opening an entirely new third park, which you would figure Disney would be responding to.
I haven't heard much concrete about DHS or DAK besides the likelihood of SW:GE expansion, but Brazil is currently lined up to open in 2023 (contracts are being finalized currently) and a new version of IllumiNations (IllumiNations: Windows on the World) is set to debut around 2022. I'm sure other things will pop up too like the Stitch replacement, Star Tours replacement, and possible expansions. Wouldn't be surprised to see a Frontierland expansion after the 50th and the future of Dinoland is up in the air as well.
I really doubt Disney is just going to go back to doing nothing after the 50th. Maybe not as much, but they will still have a motive to keep adding, especially if Universal is opening a new park as I mentioned.