I am thinking of going max damage on a natural 20; DEX save or lose
next turn on a natural 1.
For fumbles it's been "lose your next attack" for many years - the how
it's happened have always related to the circumstances. I've had
people fall, go off balance, drop their weapon, have a pole-arm
entangled in a chandelier - you get the picture. I tend to shoot for
"What's the most entertaining thing that could happen?" and my groups
have been happy to play along, because I do the exact same thing to
monsters. (For example, a orc that was charging full-steam at a PC,
axe raised and blood-curdling war-cry erupting from his maw, rolled a
1 and wound up tripping on a loose tile, smacking into a wall and
becoming stunned for a round.)
But that's a matter of trust, and having players who weren't hung up
on requiring absolutely standard and fixed rules for all happenings.
- John