Think of a coin with fixed supply, but inactivity is punished: Every X number of blocks, a wallet that hasn't been active in some way (i.e. mentioned on the blockchainfor in one of those blocks, either by transactions or staking) loses a small percentage of it's holdings, until a certain minimum amount of dust is reached, which vanishes after the next X blocks, leaving the wallet empty. The amount lost will be paid out to those who stake.
Of course, this would have a lot of implications, some good, some bad:
- People would stake just to keep their coins from disappearing.
- Cold wallets wouldn't really be a thing.
- You probably wouldn't need transaction fees.
- Coins won't get lost in wallets whose privkeys are unknown.
- …
Cold wallets could lease unspendable balance to an on-line wallet - this would improve security of the network without disadvantaging folk who genuinely need to keep large balances off-line for obvious reasons. You could limit the lease period to 30 or 60 days to stop the coins being left on a dvd+r and ensure the owner 'engaged with the network' periodically. You could also make the lease a 'one to one' deal - ie The hot wallet with public key A can only stake leased coins from one cold wallet with public address B.