This would allow wallet operations like funding transactions with smart contract-based inputs such as LN, or any input the Core wallet doesn't know how to sign for.

https://github.com/bitcoin/bitcoin/pull/17211#issuecomment-933674310
{"lock_unspents", RPCArg::Type::BOOL, RPCArg::Default{false}, "Lock selected unspent outputs"},
                    {"psbt", RPCArg::Type::BOOL,  RPCArg::DefaultHint{"automatic"}, "Always return a PSBT, implies add_to_wallet=false."},
                    {"subtract_fee_from_outputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Outputs to subtract the fee from, specified as integer indices.\n"
                    "The fee will be equally deducted from the amount of each specified output.\n"
                    "Those recipients will receive less bitcoins than you enter in their corresponding amount field.\n"
                    "If no outputs are specified here, the sender pays the fee.",
                        {
                            {"vout_index", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "The zero-based output index, before a change output is added."},
                        },
                    },
                },
                FundTxDoc()),
                "options"},
        },
        RPCResult{
            RPCResult::Type::OBJ, "", "",
                {
                    {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
                    {RPCResult::Type::STR_HEX, "txid", true, "The transaction id for the send. Only 1 transaction is created regardless of the number of addresses."},
                    {RPCResult::Type::STR_HEX, "hex", true, "If add_to_wallet is false, the hex-encoded raw transaction with signature(s)"},
                    {RPCResult::Type::STR, "psbt", true, "If more signatures are needed, or if add_to_wallet is false, the base64-encoded (partially) signed transaction"}
                }
        },
        RPCExamples{""
        "\nSend 0.1 BTC with a confirmation target of 6 blocks in economical fee estimate mode\n"
        + HelpExampleCli("send", "'{\"" + EXAMPLE_ADDRESS[0] + "\": 0.1}' 6 economical\n") +
        "Send 0.2 BTC with a fee rate of 1.1 " + CURRENCY_ATOM + "/vB using positional arguments\n"
        + HelpExampleCli("send", "'{\"" + EXAMPLE_ADDRESS[0] + "\": 0.2}' null \"unset\" 1.1\n") +
        "Send 0.2 BTC with a fee rate of 1 " + CURRENCY_ATOM + "/vB using the options argument\n"
        + HelpExampleCli("send", "'{\"" + EXAMPLE_ADDRESS[0] + "\": 0.2}' null \"unset\" null '{\"fee_rate\": 1}'\n") +