I'm slowly working on some big plans. If I understand correctly, making nostr pubkeys the same size as eth pubkeys would also make them work as Monero addresses. Ideally there would be 3 sizes of public key code: a small code containing the short key binary (current npub), a medium code containing the long key binary (like current eth or xmr pubkeys), and a long code containing both. Unless it's possible for a public key to simply be shortened and work with the private key without needing to be separately generated at the separate length, idk
Discussion
nostr:nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpqwamvxt2tr50ghu4fdw47ksadnt0p277nv0vfhplmv0n0z3243zyqkmdjma nostr:nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpqsatan3ugmhaq9wgs26tp4gdua5gslfaazut27f2qclgp82knxljsmmake6 the keys are the same length however, my understanding is that it is extremely dangerous to use the same key data in two different ways.
An easy way around this is, just create a new Nostr event that uses your regular nostr key to sign a message indicating a cryptographic proof of your ethereum key. then you just have two separate keys but they are provably linked
Unless I'm misunderstanding your point, nostr already has a key-reuse issue that can be solved at the same time by getting people to use signer apps, signer devices, and a key-transfer mechanism like you described.
But the key transfer mechanism should only be needed for migrating between devices or device groups. The normal functionality would come with the signer app storing the private key and using it to deal with all the interactions with the different public keys.