I don't think a Bitcoin wallet will need to be integrated into the OS itself. As long as apps can access a Nostr signer object they can use sign_psbt (so yes, nostr-based wallet). Through the same method they can do Cashu (NIP-60) as well. But I've also been itching to add an hkdf() method to Nostr signers so you can do things like derive an HD wallet seed from your nsec without needing direct nsec access. That would let you have a portable HD onchain wallet for Bitcoin and any shitcoins, plus you wouldn't lose your Cashu key either.