Adding 3PP isn't that simple.
First, as a platform owner, your brand is on the line if something problematic comes up. So all 3pp material would need to be checked for anything that might be problematic for the D&D brand. Second is negotiating business deals - what percentage of sales goes to WotC, what goes to 3PP creator, what are conditions ( as in can one side unilaterally decide to remove content from the platform for example). In short, it's similar to adding apps to Apple store.
@Reynard
Adventures and settings with some new player options and monsters. Stuff that appeals to broadest possible audience. They have new half edition core books that will no doubt be good sellers. Then it's back to above mentioned.