To add to what nostr:npub1t0nyg64g5vwprva52wlcmt7fkdr07v5dr7s35raq9g0xgc0k4xcsedjgqv said, one example would be to take a BDK based wallet app and integrate secure simplex messaging to pass around PSBTs for signing multisig transactions, or to collaborate on setting up and signing payjoin or coinjoin transactions.