So here's one that I've been using at my table. It sounds like more of a radical change than it is, but borrowing from other TTRPG's I wanted to see if I could make 5e function with the players rolling to defend, rather than the monsters rolling to attack.
The system came out being really intuitive for players, and since it's a 1:1 conversion it doesn't change any of the underlying math that makes the game run so it's fully compatible with other rule supplements.
I've passed with one around for a little bit and so far the folks that I've heard who tried it in their games loved it. It takes a bit of burden off of the DM, let's the players roll more dice when it isn't their turn, and speeds the the turns in combat... so here it is!
- from the publisher's blurb