In 1e and 2e we house ruled that that a 20 was an automatic success and rolled on the original critical hits and fumbles tables in the old Dragon magazine, and a 19 was a fifty percent chance to hit.
In 3.5e, some weapons scored a possible crit on a 20, others on a 19 - 20. If you had a keened weapon that increased the chance and with the right enchantments you can further increase the chance to score a critical hit all the way fown to 15 - 20.
So it's not really a new idea.
But one I still like.
Random dice rolls can be quite interesting at times because luck can go really good or really bad.