Miscellaneous ideas...

Some portion of the hash rate operating over mesh networks. Or at least a fallback when internet service is inaccessible. If any miners were to lose internet they automatically find the closest miner still with access and piggy back. Connecting over bluetooth?

Mining on cell phones.

Bitcoin road tolls. Maybe a 2of2 multi sig wallet that has time locks and automatically transfer coins when they come within a specific range for specific time frame. One signer remains in the vehicle while the other is a mounted above the road. It could communicate over bluetooth.

Could be interesting in other scenarios. Basically to cell phones with bluetooth turned on. If they come in to range of one another they automatically a transaction.

Simple app could manage the agreed upon transaction size and refresh rate.

Geographic center of hash tracker?

Simple chart of the estimated kyc percentage of supply.

Currently not applicable but in the future, an attempt to estimate the actual human population using only on chain metrics.

A robust way to time lock coins long term for example kids retirement fund, 50 year minimum and the address containing the coins must not be visible until unlock date. So that the value is unknown. Maybe even hide the unlock date if possible? Might be easier to build a physical solution.

May already exist. A simple app for landlords to manage and receive pre approved rent. Again maybe the coins are time locked, payments scheduled automatically for 12 months at a time, that way the tenant can hold as long as possible well the landlord can be 100% confident the funds will be available. Landlord could receive automatic warnings if agreed upon wallet removes btc. The app should be able to easily manage hundreds of such arrangements.