glass said:
I've long thought that Keen should add to the multiplier. Mathematically identical to stacking on the threat range (almost), but still keeps crits 'special'.
glass.
Increasing the multiplier is not a fair ability in a world with uneven critical ranges. Why not? You must first agree that the worst critical is 20/x2, which is worth an arbitrary "1" critical point (1 chance in 20 at adding one extra amount of damage). 20/x3 and 19-20/x2 are worth 2 points (2 chances at adding one extra amount == 1 chance at adding two extra amounts), and 20/x4 and 18-20/x2 are worth 3. WotC knew this even back in 3.0, and they have balanced weapons assuming this (typically a critical "point" has cost a weapon a lower damage dice).
Okay, so why does this matter? Well, the awesome thing about the way Improved Critical and Keen work is that they preserve the partial order of the weapons' critical points. In English? Weapons that have equal critical points before Keen have equal critical points after Keen, weapons that have less before have less after, etc.
Now, there are two dangerous things you can do that seem harmless but actually do not balance at all with the basic balance.
Thing #1: Just add a flat +1 to the threat range. Some people have mentioned that in this thread. Why isn't it balanced? 20/x4 weapons become 19-20/x4 and go from 3 critical points to 6 critical points, but 18-20/x2 only go from 3 to 4. Even 20/x3 go up to 4, making the Battleaxe better than the Rapier and with higher damage.
Thing #2: Increase the critical multiplier by 1. Why isn't it balanced? Opposite reason. 20/x4 weapons become 20/x5 and go from 3 critical points to 4, while 18-20/x2 becomes 18-20/x3 with 6 critical points. Even Greatswords with their 19-20/x2 become 19-20/x3 and have 4 critical points, equal to a Scythe.