Two things:

a) Human beings are not good at picking random phrases so don't create your own brain wallet passphrase.

b) Don't reinvent the wheel - use electrum

Electrum generates a random seed that is basically 12 words. You can memorize those or write them down. They are all you need to restore your wallet with multiple addresses. Electrum also supports watch only wallets and offline wallets. So if you want to create a brain wallet use electrum. It is all you'll ever need.