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.
const CTransaction txConst(*psbtx.tx);
    
    bool bip32derivs = request.params[4].isNull() ? false : request.params[4].get_bool();
    FillPSBT(pwallet, psbtx, &txConst, 1, false, bip32derivs);
    
    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
    ssTx << psbtx;
    UniValue result(UniValue::VOBJ);
    result.pushKV("psbt", EncodeBase64((unsigned char*)ssTx.data(), ssTx.size()));
    result.pushKV("fee", ValueFromAmount(fee));
    result.pushKV("changepos", change_position);
    return result;
}
UniValue abortrescan(const JSONRPCRequest& request); 
UniValue dumpprivkey(const JSONRPCRequest& request); 
UniValue importprivkey(const JSONRPCRequest& request);
UniValue importaddress(const JSONRPCRequest& request);
UniValue importpubkey(const JSONRPCRequest& request);
UniValue dumpwallet(const JSONRPCRequest& request);
UniValue importwallet(const JSONRPCRequest& request);
UniValue importprunedfunds(const JSONRPCRequest& request);
UniValue removeprunedfunds(const JSONRPCRequest& request);
UniValue importmulti(const JSONRPCRequest& request);
static const CRPCCommand commands[] =
{ 
    
    { "generating",         "generate",                         &generate,                      {"nblocks","maxtries"} },
    { "hidden",             "addwitnessaddress",                &addwitnessaddress,             {"address","p2sh"} },
    { "hidden",             "resendwallettransactions",         &resendwallettransactions,      {} },