I've only done 2 homebrews - full classes, because, well, you get to build entire mechanics!
First one was for a Homebrew Magepunk setting. It's had various names but currently lives under the title of
The Arcanist. It's half melee Ranger, half Tactical Fighter, and is probably closest conceptually to a Barbarian. Think "Magic Ghost Swordsman" and you're sort of there. It was explicitly intended as a STR/INT build, so I've done some fun stuff with decoupling AC from Dex which took about 40 revisions to get working....I think it's different. There's 2 subclasses: One focuses on Melee damage and becoming more Ghost-y, the other focuses on versatility and is more Ranged combat & magic focused (sort of like if the Artificer was a Marine Soldier). While levelling, I tried to leave the core mechanic alone and add more roleplay and fluff options, so the essence of the class (being able to turn into a Magical Ghost) was thematically more applicable.
Second one I did was a vanity project. I loved the Warden from Lord of the Rings Online - The gambit system was so fresh and interesting, and a Tank the tanked via regeneration was really clever as a concept. So I kinda did my own rebuild of that and called it
The Warden for want of a better name. The Core mechanic there is almost identical to the LOTRO Warden: Make different attacks, build a Combo, Initiate Combo, rinse repeat....I also deliberately hung it on a Wizard Chassis, but buffed shield usage for higher AC from STR and added in Self-Heal options so, at a push, The Warden can switch from Focusing on Damage to being more of a Defender (occupying monsters) while recovering. Level progression is almost entirely built around building better Combos, or Roleplay options. The 3 Subclasses focus on Melee attacks, Combo Attacks, or Healing/Protecting allies.
I've only really managed to get one Arcanist on the table, and that's quite RP heavy, so not really seen how capable it is in a straight dungeon crawl. But I think they are fun concepts and mechanics (obviously I would, we all love our ugly children) - just a shame its so hard to get feedback.