What if the brain wallet is just a memorable private key itself? A brain wallet usually applies a hash function on a passphrase to derive the private key. But what if the private key itself is memorable and we just use the private key as the brain wallet?

For example it is very easy to memorize this private key:

Private key: 5JustSomeVeryEasyToRememberBitcoinPrivateKeyWALEmag
Address: 1GtAW7vntpij1q8Mqi3FQw8R1gAd9rnRV4
A series of random words (not a sentence!) long enough to use as a secure key should be about 12 words long, which could be around 100 characters long (figuring they're fairly long, 7-8 average, with a space in between). This gives you 2^128 combinations that you might have to try to crack your password, which might take about 1 billion billion years.

A private key, in base 58 format, is 51 characters long. Your idea of using a 51-character sentence as a private key would probably give about 51 bits of entropy (English is around 0.6 to 1.3 bits per character), which could be cracked in about 12.5 seconds.