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.
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() < 2 || request.params.size() > 8)
        throw std::runtime_error(
            "sendmany \"\" {\"address\":amount,...} ( minconf \"comment\" [\"address\",...] replaceable conf_target \"estimate_mode\")\n"
            "\nSend multiple times. Amounts are double-precision floating point numbers.\n"
            + HelpRequiringPassphrase(pwallet) + "\n"
            "\nArguments:\n"
            "1. \"dummy\"               (string, required) Must be set to \"\" for backwards compatibility.\n"
            "2. \"amounts\"             (string, required) A json object with addresses and amounts\n"
            "    {\n"
            "      \"address\":amount   (numeric or string) The bitcoin address is the key, the numeric amount (can be string) in " + CURRENCY_UNIT + " is the value\n"
            "      ,...\n"
            "    }\n"
            "3. minconf                 (numeric, optional, default=1) Only use the balance confirmed at least this many times.\n"
            "4. \"comment\"             (string, optional) A comment\n"
            "5. subtractfeefrom         (array, optional) A json array with addresses.\n"
            "                           The fee will be equally deducted from the amount of each selected address.\n"