GWM is actually a great reference for them to work with and expand on since it's already in the game. Cleave &co could easily follow that design pattern.
It's a much more D&D thing to do to counter the attack penalties by casting Bless on your Fighter, and letting them loose at the enemy to Cleave and Bash as much as they like.
The only thing I don't like about this is that GWM is an entire feat. So these weapon-abilities shouldn't be nearly as powerful as it; otherwise I definitely agree that they should be class abilities.
But Cleave: -5 to hit in exchange for making 2, maybe 3 attacks, seems roughly balanced. It will work out to a very situational ability that, on average, only grants slightly more damage when used.
GWM deals a lot more damage to a single target and also allows bonus action attacks, so is still much more powerful.
