Why not all of the above? 
That sounds cheeky, but hear me out…
Something Drupal has the notion of is “distributions” which are essentially various starter kits with some configuration and add-ons pre-installed: Drupal for Commerce, Drupal for Social, Drupal for Publishing, Drupal for Government, etc.
So what if we pulled your awesome idea up a level, we could potentially create a whole new category of thing, “MongoDB Starter Kits” or what have you… outline some guidelines/templates to follow, and then invite the Community (capital “C,” inclusive of MongoDB internal folk as well) to contribute these.
Then the submissions could be inclusive of both pragmatic, solve X problem things (Quiz, Twitter clone), as well as more “framework-y” things (Form Builder), and even more niche interests like a nature protection project. All would have a place.
Sorry, not meaning to add scope. Just putting forth the idea that while you’re figuring out which of these ideas to work on first (which you have to do either way), also maybe try and think about what infrastructure would need to be in place to have 20 or 30 of them. I’m super happy to help with brainstorming this, if you think it’s a good idea. 