I like the idea of a single die-roll though, without having to mess about with modifiers and what-not, and I thought that since we have all these funny-shaped dice, why not make use of them?
To that end, I decree that the actual die rolled for surprise will depend on the character class, species, and possibly level. The default die remains the d6.
- Naturally alert critters like elves, barbarians, rangers, watch-dogs etc. are only surprised if they roll a 1 (16.6%).
- Very alert critters like ettins, mid-level barbarians or rangers etc. get to roll a d8 for surprise instead of a d6, still requiring a 1 to be surprised (12.5%).
- Extremely alert critters or mid-high level barbarians/rangers get to roll a d10 (10%)
Things like wearing a vision- or hearing-restricting helm would drop your surprise die down one or two levels, so a fighter wearing a closed armet would be rolling a d3 for surprise (66.6%)
Now, all of this palaver could be sorted out easily by converting the surprise mechanic to a d20 or d% roll, but that has at least three strikes against it:
- It makes it more difficult to adjudicate the effects of monster surprise descriptions, like those that surprise on a 1-5, and
- It makes the "number of segments of surprise" thing more difficult to work out, and
- It doesn't use the whole range of funny-shaped dice.