Thought I would share an idea reaped from Hackmaster 4e (my principal ongoing source of AD&D inspiration) that I have ported into my 5e game. The HM 4e GMG has a brilliantly crunchy and awesome d10,000 critical hit table, complete with anatomical maps.
My players absolutely love criticals, I mean don't we all ... But we have quite a bit of fun with them. One player started a tradition of playing a "Critical Hit!" gif on their phone, (no, not the one above, that is just my geeky love of Jerry Lewis, may he rest in peace) and of course they all groan in doom laced unison when somebody fumbles. We've used standard double damage, maximum damage rules for Crits, and the 1-2 drop, 3-4 break, 5-6 self inflicted wound for fumbles; we also used the Pathfinder Critical Hit and Fumble decks; and finally the HM d10,000 critical hit tables. The HM tables have been the favorite by far. I a in the process of modifying them, as they are not exactly designed for a d20 based game, and I'll share my tables as soon as they are complete. But you can find the originals in the HM 4e GMG. They also use a similar and somewhat simpler table in 5e, but it is based on their new two roll combat system--which you could also modify. I've wanted to get the 2e Players Option book on combat to get an idea of what inspired HM, but haven't had the time. The one complaint I have about the tables is that I have not found a way in the rules to modify them for animals other than basically human-like in anatomy. Regardless, they tables are a great success at the table, and only take about 2 minutes to apply--definitely worth it for a critical. And believe it or not, I think rolling the d10,000 is their favorite part:
We also use the fumble table from HM 4e, and they are pretty cool, though not quite as varied as the crits, you only roll a d1,000. But nonetheless, better than anything we have used thus far... For, no matter how awesome the descriptions I weave in for the crits, nothing brings laughter to the table like a well played fumble