A dev is already building a NXT client with wallet.dat approach (forgot which one). You generate a password, and the client stores a long and secure private key locally, encrypted using your password.
If a hacker wants your account, he would have to break into your system and steal your wallet.dat plus your password. Similar to bitcoin.
In NXT, both methods are possible: Only brainwallet, or brainwallet plus local wallet.dat
Please everyone note the following: This will be a tradeoff. If your Notebook explodes and you didn't backup wallet.dat, all your NXT will be gone. The brainwallet approach NXT uses gives you the possibility to be independent from a wallet.dat. If your house burns down, you can still acces your NXT, if you can remember a long password