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 (data.exists("timestamp")) {
        const UniValue& timestamp = data["timestamp"];
        if (timestamp.isNum()) {
            return timestamp.get_int64();
        } else if (timestamp.isStr() && timestamp.get_str() == "now") {
            return now;
        }
        throw JSONRPCError(RPC_TYPE_ERROR, strprintf("Expected number or \"now\" timestamp value for key. got type %s", uvTypeName(timestamp.type())));
    }
    throw JSONRPCError(RPC_TYPE_ERROR, "Missing required timestamp field for key");
}
UniValue importmulti(const JSONRPCRequest& mainRequest)
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(mainRequest);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, mainRequest.fHelp)) {
        return NullUniValue;
    }
    
    if (mainRequest.fHelp || mainRequest.params.size() < 1 || mainRequest.params.size() > 2)
        throw std::runtime_error(
            "importmulti \"requests\" ( \"options\" )\n\n"
            "Import addresses/scripts (with private or public keys, redeem script (P2SH)), rescanning all addresses in one-shot-only (rescan can be disabled via options). Requires a new wallet backup.\n\n"
            "Arguments:\n"
            "1. requests     (array, required) Data to be imported\n"
            "  [     (array of json objects)\n"
            "    {\n"
            "      \"scriptPubKey\": \"<script>\" | { \"address\":\"<address>\" }, (string / json, required) Type of scriptPubKey (string for script, json for address)\n"