That's like asking what flavors we'd like to see mixed with water.

The bitcoin ecosystem is MONEY. We want to do everything with money, not just one killer app; absolutely everything.

Now if you're asking what is sorely missing right now I'd suggest some kind of extremely simple marketplace that any 6th grader could set up with their android phone that sells things for sats with no kyc. -And share the link around to their fellow students.

Good luck!