A few notes first...
I haven't play-tested these at all, so they're entirely theoretical. It's changes I like, so I'll probably make use of them when I use the creatures in 5e and update the posts with any notes or discoveries.
I'll probably steal liberally from 13th Age and Dungeon World. I also plan on only looking at monsters from the D&D Basic rules so that the originals are freely available for comparison.
- Trigger: Natural even miss
- If the target is engaged with at least two opponents the target takes 1d8 damage on the miss.
- Great Flail
- Melee Weapon Attack
- +4 to hit, reach 5 ft., one target.
- Hit: 13 (2d10+2) bludgeoning damage
- Natural 16+ : target is tripped and knocked prone
- Nuthin' but the hide armor on his back and the weapons in his hands.
- 2d6+X Silver Pieces, where X=# of bugbears, like 2d6+8 SP for 8 bugbears
- 2d6xY Copper Pieces, where Y=# of bugbears, like 2d6x4 CP for 4 bugbear
- Roll twice on a trinket table (like this one or this one)
- 1d6 Gold Pieces
- Poorly smoked meat jerky, equivalent to 1 day of rations, but PCs who consume it need to make a DC 5 CON check or be Poisoned for 1d4+1 hours.
- 10' of chain
- a Hunting trap
- A crude map of the best ambush locations in an area (probably the combat location, but if the DM is feeling generous and/or the bugbear fight was tougher than expected it could be a different nearby location)
- A poisoner's kit
- A single use magic item, like off this D&D 5e Cypher generator or the Sly Flourish Single Use Relic generator.