With cryptocurrencies like bitcoin, you must guard the private keys which give you control of your own bitcoins well. You must guard them from being stolen (which calls for secure wallet programs, strong passwords and even offline paper wallets) but you must also guard them from being lost, which means backups, and possibly escrow in the event of your death.

These two desires can sometimes be at odds. For better or worse, there are reports of people who have lost many thousands of bitcoins, today worth many millions, due to hard drive failures, poor backups and other reasons. They are gone forever.

A coin could be created with this rule. Every coin must be "spent" in a transaction every so often, such as every 2 years. You can "spend" your coins by sending them in a transaction back to yourself, usually to a new unknown address. Any coins not spent for 2 years would be declared lost, and would be transferred to the address of a charitable foundation created by the coin's creators. The foundation would have a charter promoting suitable causes -- which could range to cryptocurrency related causes to other popular ones. It probably would not be a controversial or polarizing cause because people might be annoyed if the coin is seen benefiting something of that sort.

All wallet software for this coin would be programmed to make sure all coins were spent in such a way on a regular basis. This would incur small transaction fees, but they should be quite minor. (If desired, the rule could be that small fragments, which are not at least 1000x expected transaction fees, would not be subject to this expiration rule. Or there could be a practice that renewal transactions are marked as such and usually processed without fees. In the long run, I expect fees to get quite low anyway.)

This would mean you could not keep coins long term in paper or offline wallets.

Existing blockchains

Existing blockchains could try this approach as well. The least controversial way (though still controversial to be sure) would be to declare the policy in a new software revision, only for addresses created after the revision, and only after there was time for all wallets to adapt for the requirement to renew.

Much more controversial -- but most valuable for charity -- would be a retroactive declaration. In that case, the declaration would not take effect for several years, telling people, "If you don't refresh your coins by spending them to yourself sometime in the next 3 years, they go to the foundation."

The value of that proposal is that a lot of the lost coin is early bitcoin. There are known reports of major losses. There are many people who believe that Satoshi him/herself may have lost a large block of coins disk destruction. There are also those who believe that Satoshi is dead. (They largely believe this because Hal Finney is a leading candidate for being Stoshi. Hal is cryopreserved, which means the keys to his bitcoins might exist in his preserved brain for possible future extraction with as-yet nonexistent technology, or of course his reanimation. I did not know Hal well but he did not die the death of a billionaire, that is for sure.)

Even so, it can't be ignored that even without Satoshi's $6B worth of bitcoin, there is probably hundreds of millions of dollars of lost bitcoin out there, and it's a shame that it should just stay destroyed, rather than doing good in the world. Of course, it can be pointed out that the lost bitcoin shrinks the supply of traded bitcoin, and thus increases the value, so the value is not lost. That's partially true, but not entirely, unless everybody knows the bitcoin to be truly lost.

And yes, there are some who have lost bitcoin that might be found in the future. I have a paper bitcoin somewhere that I can't find. I would lose it under this scheme. There may be hard drives sitting in storage units, or dead drives that can be recovered with future technology. There are supposedly drives in landfill somewhere with significant coin on them. But the odds of recovery are low and the benefit good.

Even so, I doubt a retroactive proposal would win the world. This doesn't mean that future coins or future forks of coins might not, so I put it out there.