An idea formed after posting and reading comments on one of my vacation videos here. Rob Couture had the original idea, FYI. His YouTube comments were lost when merging his two accounts.
Basically, every round of combat increases the critical-hit range by one.
On the first round, the vast majority of PCs and monsters will only crit on a natural 20. On the second round, they'll crit on a 19 or 20. On the third round, the crit range is 18-20. Fourth, 17-20 and so on...
Critical-hits do the usual double damage, and it also means you definitely hit the target. This is useful especially for high AC opponents when, on the fifth round, you will hit on anything that's a roll of 16+.
If you're fortunate enough to have a higher than normal crit range, then good for you. You get to keep your +1 crit range bonus, that just makes you a more efficient killer!
Regardless of the crit range, if a natural 20 is rolled, that character may also perform some sort of stunt, special maneuver, or epic deed based on present circumstances, described by the player (if he wishes), and adjudicated by the Game Master.
This will shorten combat and also make it more deadly. Two things the OSR usually prefers over modern D&D. I'm going to officially add this to Crimson Dragon Slayer D20 soon. Until then, I'd like you guys to help me playtest it.
I don't see a potential downside, assuming fast and brutal battles is what you want, but you never know. Additionally, this should help balance out non-sorcerer classes, as well. Suddenly, those wading into melee are slightly more awesome than they were before. Take that, fireball spamming wizards who's best friends with a cleric!
p.s. We're between Cha'alt kickstarters at the moment, but if you want your luxury supreme hardcovers, Order them right now!