A derivation scheme similar to BIP44 for the upcoming P2WPKH-nested-in-P2SH addresses.
The test vectors aren't finished yet, as I choose to have the BIP-number as base path (like BIP44) in it. I'll update them as soon as this gets a preliminary number assigned.
Fix url for BIP44
Looks like Ledger is using this: https://github.com/LedgerHQ/ledger-wallet-chrome/blob/master/app/src/wallet/wallet.coffee#L76-L80 @amougel it might be useful to add that to the (new) list of implementations.
This is different form how the addwitnessaddress RPC call works in Bitcoin Core, which uses the same private key. It even allows you to create a witness address for a private key when its "legacy" address has received funds. I don't know what the plan is future wallet UI.
Although the core wallet (currently) doesn't use BIP 44, it's still useful to note how it handles SegWit vs. "legacy" key derivation, as well as private key reuse. Perhaps that can be added to the BIP.
Trezor is also using m/49'/0'/0'/0/0 (=first account/first receiving address, currently only at https://beta-wallet.trezor.io)