I see coins aging could be a viable on-chain scaling solution (scenario where coins that was not been moved for full halving period will automatically return to coinbase). If this will be the case - full nodes will only require to store blockchain for the past halving period since anything that remained unspent will transfer to the coinbase. This is drastic and, perhaps, cruel measure to take but that will also solve satoshi't billions, burned and lost coins problem.

Here are a few of the ideas which I think would be most interesting to see in an altcoin. A few of these things may be possible as hardforking changes in Bitcoin too but some represent different security and economic tradeoffs and I don't think those could be ethically imposed on Bitcoin even if a simple majority of users wanted them (as they'd be forced onto the people who don't want them).

(Some of these ideas are mine, some are from other people, some are old and obvious)

[...]

• UTXO aging
  
  • Abandoned UTXO should be forgotten and become unspendable.
  • Demurrage is one possible construction for this, but its economically and educationally complicated. Simpler would just be having a long but finite maximum. Unspendable coins could vanish forever, or be returned to mining— but returning the coins to mining is economically distorting and risks creating weird incentives ("I won't mine your txn because I want to collect its inputs as fees once you've been successfully denied")
  • ATTENTION MORONS: THIS CANNOT BE DONE WITH BITCOIN. SEE THE LARGE BOLD TEXT AT THE TOP.