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() > 2) {
        throw std::runtime_error(
            "generate nblocks ( maxtries )\n"
            "\nMine up to nblocks blocks immediately (before the RPC call returns) to an address in the wallet.\n"
            "\nArguments:\n"
            "1. nblocks      (numeric, required) How many blocks are generated immediately.\n"
            "2. maxtries     (numeric, optional) How many iterations to try (default = 1000000).\n"
            "\nResult:\n"
            "[ blockhashes ]     (array) hashes of blocks generated\n"
            "\nExamples:\n"
            "\nGenerate 11 blocks\n"
            + HelpExampleCli("generate", "11")
        );
    }
    int num_generate = request.params[0].get_int();
    uint64_t max_tries = 1000000;
    if (!request.params[1].isNull()) {
        max_tries = request.params[1].get_int();
    }
    std::shared_ptr<CReserveScript> coinbase_script;