I'd love to see a really good starter adventure set. Something along the lines of Phandelver, with a good central location for players to get connected to. Not world spanning, just a local threat that can be built on into a larger homebrew campaign, or extended through a second adventure released separately. I'd be happy for this starter adventure to be level 1-5 if they can really deliver on the next paragraph.
As important as the adventure side would be to build this as an on-boarding game for new DM's. Really build it out with advice on how to run a campaign through the adventure. Start with Session 0 tips, then into simple encounters, dungeons, how to run npcs, home bases, foreshadowing a villain, escalating the story, providing rewards, introducing quests, etc. I think Lost Mines of Phandelver and the newest Starter Set both do this reasonably well, but I think it could be done even more intentionally and stronger.
Make this starter set truly the essential, best way for a new DM and set of players to get into the game.