My comment isn't to pass judgement on your suggestion, but if you took a look at homebrew itself you'd be able to make better non-generic suggestions.

Re: your suggestion - It's a generically seemingly good thing to separate out fetch from install, but as a user of homebrew, it's not very applicable because when homebrewing things you're likely already connected to the internet, and it's hard to predict when you want to brew install something before hand. If you have the internet capacity to fetch something, it'd be just as easy to brew install it there on the spot.

By extension, it's important to run brew update before installing just to make sure the package index is up to date, so I agree with the dev above, integrating brew update step before brew install would be a good thing - except - perhaps print out on console the exact version number that's going to be installed. Current behaviour does put the version # in the file name of the package being installed, but it could be listed in a more obvious way.

Often times I do a brew info, find the version and details on it before brew install. If the installation step then installs a new version (because of the brew update step), then it's a bit strange that I didn't get the version I was intending to get.