Grappler;
I allow the PC with the Grappler feat to Throw/Slam enemies to the ground. Either make a Shove attack, or roll to Pin/Takedown/Slam/whatever from the Grapple. PC rolls d20+prof bonus+Athletics bonus, attack does a 1d4+STR mod.
My reasoning is that, if a Tavern Brawler can do a d4 worth of damage by slamming a fist into someones head, a Grappler should be able to do the same amount of damage by slamming someones head into a floor, wall, etc.
Mounted Combatant;
Anyone with this feat can have their mount make one attack as a bonus action. Attack uses the Mounts stat block for attack rolls and damage. My reasoning is that any horseman worth his salt can get his mount to kick, bite, etc. Also, it might be a bit OP at times, but this is balanced out by the fact that mounted combat is relatively rare in most campaigns.