In the spirit and since basically decrypting the keystore to extract private key(s) to derivate addresses is just one step more than this, can we also provide a way of importing a private key directly?

It might be more rare, but very early wallets and many paper wallets were just a single, unencrypted, private key.

Wallets like MEW (MyEtherWallet) have this ability for example: https://www.myetherwallet.com/wallet/access/software?type=private-key