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() < 1 || request.params.size() > 4)
        throw std::runtime_error(
            "importaddress \"address\" ( \"label\" rescan p2sh )\n"
            "\nAdds an address or script (in hex) that can be watched as if it were in your wallet but cannot be used to spend. Requires a new wallet backup.\n"
            "\nArguments:\n"
            "1. \"address\"          (string, required) The Bitcoin address (or hex-encoded script)\n"
            "2. \"label\"            (string, optional, default=\"\") An optional label\n"
            "3. rescan               (boolean, optional, default=true) Rescan the wallet for transactions\n"
            "4. p2sh                 (boolean, optional, default=false) Add the P2SH version of the script as well\n"
            "\nNote: This call can take over an hour to complete if rescan is true, during that time, other rpc calls\n"
            "may report that the imported address exists but related transactions are still missing, leading to temporarily incorrect/bogus balances and unspent outputs until rescan completes.\n"
            "If you have the full public key, you should call importpubkey instead of this.\n"
            "\nNote: If you import a non-standard raw script in hex form, outputs sending to it will be treated\n"
            "as change, and not show up in many RPCs.\n"
            "\nExamples:\n"
            "\nImport an address with rescan\n"
            + HelpExampleCli("importaddress", "\"myaddress\"") +
            "\nImport using a label without rescan\n"
            + HelpExampleCli("importaddress", "\"myaddress\" \"testing\" false") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("importaddress", "\"myaddress\", \"testing\", false")
        );
    std::string strLabel;