Thanks for the links! Blocksonly is definitely a relevant piece. However, I’m wondering what are the implications, especially at larger scale. For example, transactions processing will be not smooth anymore and will happen every 10 minutes at once. Another question is transaction propagation.
I took the liberty of turning this into a BIP proposal -- the formatted version can be seen here: https://github.com/kallewoof/bips/blob/bip-typed-wif/bip-extended-privkey.mediawiki BIP: XXX Layer: Applications Title: Typed Private Keys Author: Karl-Johan Alm