Changing the Crawlers to not have guns is a fundamental change (which 343 loves) but a fun one.

I might change the watchers even more though. In your model I still think they will encourage the same style of game-play. Yes, they should not immediately run away and fold up like they do now, but they need to find a way to make them more interesting. They shouldn't be solely defense as I don't think that is really ever satisfying to kill with a gun. It is however...fun to kill with a punch.
Killing them with a punch is a little out there though.