It's currently not possible to import (solvable) SegWit addresses (P2SH and native) using `importmulti`. I believe that's a shortcoming we should address before release.

My suggestion would be for P2WPKH (P2SH and native) to be supported without any extra arguments. This means that a P2WPKH address/script with corresponding private or public key should work. For P2SH it means checking if the supplied P2SH address perhaps matches the supplied public key or private key when converted to P2WPKH first.

To support P2WSH, I think we need an extra field "witnessScript". When supplied, it would either be required to match a P2WSH address/script directly, or if the address is P2SH, indirectly.
if (scannedTime <= GetImportTimestamp(request, now) || results.at(i).exists("error")) {
                    response.push_back(results.at(i));
                } else {
                    UniValue result = UniValue(UniValue::VOBJ);
                    result.pushKV("success", UniValue(false));
                    result.pushKV(
                        "error",
                        JSONRPCError(
                            RPC_MISC_ERROR,
                            strprintf("Rescan failed for key with creation timestamp %d. There was an error reading a "
                                      "block from time %d, which is after or within %d seconds of key creation, and "
                                      "could contain transactions pertaining to the key. As a result, transactions "
                                      "and coins using this key may not appear in the wallet. This error could be "
                                      "caused by pruning or data corruption (see bitcoind log for details) and could "
                                      "be dealt with by downloading and rescanning the relevant blocks (see -reindex "
                                      "and -rescan options).",
                                GetImportTimestamp(request, now), scannedTime - TIMESTAMP_WINDOW - 1, TIMESTAMP_WINDOW)));
                    response.push_back(std::move(result));
                }
                ++i;
            }
        }
    }
    return response;
}