To recap, my basic gripe with the d20 ability check / skill system built into D&D 5 is as follows:
It introduces a very large random factor into things which, to me, don't seem that random. I'm totally cool with there being a large random element in combat (d20 + modifier vs AC) and with saving throws (d20 + modifier vs DC), but for skills it seems that the random element (d20) is way too significant. For example, the difference between the strongest person in the world (STR 20, +5 modifier) and an average person (STR 11, 0 modifier) is equivalent to only 25% of the random factor.Having considered this some more, I'm now not 100% convinced with the roll-under solution I proposed previously. I like roll-under checks a lot, especially due to the target number being written directly on the character sheet, but there are some corner cases which got me concerned about the robustness of such a system. It seems, for example, to fall apart a bit for creatures with > 20 in an ability score.
Looking at the range of numbers involved, it seems that ability check modifiers for PCs go roughly from -4 to +10 (including proficiency bonus). This means that a world-level master (+10) is 50% (of the d20 range) better than someone completely average (+0). So, how about changing the d20 into something with less of a random range? d6, say. Now a world-level master is 166% better than the average person. That sounds a little bit more reasonable to me.
How would this work with the normal DCs? Assuming about a +10 bonus at the high end of the scale, this means that adding a d6 on top of that gives us a potential very high roll of 16. The standard DCs go up to 30 (described as "nearly impossible"), so some adjusting will need to be made. A simple halving seems appropriate, resulting in the following DCs (rounding fractions down):
- Very easy: 2
- Easy: 5
- Medium: 7
- Hard: 10
- Very hard: 12
- Nearly impossible: 15
Another aspect to consider is the range of checks which very accomplished characters can now automatically succeed at. With a d20 and the standard DCs, a character with a +10 bonus could automatically perform any "easy" task, but would have to roll for others. Using a d6 instead, this very accomplished character can automatically perform any "hard" task. This also seems about right to me, considering the rarity of a +10 bonus.
Addendum: I just realised that some characters (rogues, in particular) have features which allow them to double their proficiency bonus. This would probably have to be replaced with a +1 bonus, in a d6 ability check system.