I've strangely never seen anyone mention this, but adding Ethereum logins to nostr would be easy af because Ethereum public keys are already longer than the 32 bytes for a nostr key, so you could just use the last 32 bytes of your Ethereum keys as your nostr keys.
And your Ethereum wallet address would still be automatically attached to your account because it only uses the last 20 bytes of your public key.
You wouldn't actually need to make nostr compatible with a full-fledged Ethereum wallet; letting users enter their Ethereum keys to extract the last 32 bytes would just make nostr signing apps their own "Ethereum wallets" that maybe can't handle the transacting functions of an on chain Ethereum wallet, but can use the Ethereum keys to sign nostr events.
This might also work for other cryptocurrencies, I'm not sure. Bitcoin for example seems to have the addresses range from 25 to 34 bytes