Common Heroes in AFK Arena are basically just fodder for the Rickety Cart. They can't be upgraded to anything and they're very weak.
Retiring Them
When you retire them you'll receive Hero Coins which can be used in the Barracks, and Dust which is used to level up heroes at certain level thresholds.
Using Them For Bounties
They do have one additional usage, and that's using them for bounties. If you don't have enough heroes of a specific faction yet it can be a good idea to save common heroes until you've got more than enough to dispatch all bounties at the same time without them.
Conclusion
It's a bit of a shame that they aren't more important in the game really, as they do have their own unique looks and skills.
Hero List
There's 2 common heroes for each faction. Interestingly there's no agility heroes for some reason.
We've listed them all below: