Update from the reddit thread: (dev answer in red)

What is an example of a simple dapp that would be cool?
One reason why I'm asking is I'm interested in building a dapp to drive people to Block. I can program to some degree of proficiency but I'm not on Dan's level.

I'll list a couple of simple ideas:
Zcash mixer (clone a multiwallet, integrate xbridgep2p, auto-trade for Zcash and back to any other currency. Near-perfect mixing)
A marketplace app. You'd want the following services: (a) customer reputation and info, (b) payment processing, (c) image storage, (d) item listings. Use a microservices architecture for the reasons given above, gaining the advantages of utilising multiple blockchains. Use one chain to store encrypted customer info (see the final item on this list), use the Blocknet's XBridge to accept payments in any cryptocurrency, store images on a server, and use a third chain and in-wallet code for the item listings and UX. The result: a scalable, composable set of services that are easier to bugfix, ungrade, or replace.
The use of any Ethereum contract by supplying "gas" in other coins
A stablecoin that maintains its peg by exploiting the fact that trade records on a dx are on-chain (checks trade records; burns/creates or freezes/unfreezes coins to maintain a precisely equivalent money supply)
An OAuth killer: a personal information service that records encrypted personal metadata on your chain and comes with a revocable permissioning system. Users thus acquire self-sovereignty over their personal information. From there, you can integrate to any site/app requiring sign-in, or users can voluntarily sell their metadata to advertisers for micropayments (sorry, Google Ads), or it can support passport/identity systems.