I really like the ideas behind the Warhammer dice, but I'm not sure I would have taken it as far as they did. Having so many skulls and eagles come up means you really have to really come up with a lot of Boon and Bane results. This forces a pretty crunchy system with action cards, and requires either a lot of GM effort between sessions, or the use of a published module. The latter is clearly what the publisher is pushing towards, but it's never really been my style. I've tried trimming it down a bit for Everhammer, which worked pretty well. I've also tried trimming it down rather further than that, for WhamTrek, which didn't work as well mechanically but was silly enough to function for a one-shot. In the lite narrative version, it felt like boons and banes were happening way too often - every single die roll resulted in the GM having to come up with something cool and unexpected. It was draining.
One of the other problems I have with the Warhammer dice is that it's not symmetrical. (Peter pointed this out to me the other day.) If I roll Observation opposed by your Stealth, in Warhammer's system my Stress or Fatigue go up or down by the results of the roll, but your stress or fatigue is always unaffected. As a GM, I plan to roll dice a lot less often than my players do (I prefer player-facing systems), but this particular effect of the Boon & Bane system makes that plan a tiny bit undesirable.
My players the other night got me thinking about the Wild Die in d6 Star Wars. Overall though, if I recall correctly, it didn't quite do what I wanted, either. The benefit of rolling a "6" was that it increased your success rate instead of adding beneficial side effects. The not-strictly-numerical complications and side effects of a "1" only happened in one roll in 36, and that just wasn't often enough to be interesting. Making them happen 1 in 6 rolls would likely be too much, though.
What I think I want in an RPG mechanic is something that works like boons and banes, but a lot less often, maybe 30% of all rolls having some sort of side effect either good or bad (instead of 60% or 70% of rolls like it is in Warhammer 3rd). It would also have a few less levels of gradation, too - I'd like to be caring about 1 to 2 boons or banes on a roll, not have to be prepared for oddball rolls like "4 boons and 1 chaos star".
I was contemplating this the other day, and it occurred to me I could handle this, in any tandem with just about any game system, by using a single blank d20. I know chessex makes large blank d20s you can mark up with a sharpie. I'd prep this extra die, and use it in conjunction with a more normal success/fail mechanic of whatever game I was running. As an example, you could add exactly one die like this to the default dice pools of World of Darkness. Or, you could roll this die along side a normal d20 in D&D to trigger exceptional events that are more interesting than just double-damage.
I'd draw on 6 sides of it (so 30% chance of something unusual happening), with a total of 4 symbols. Probably this distribution:
- 2 sides marked with a 4-leaf clover - "Lucky Break" - These would result in something unexpected and good happens to the character. These would be improvised, or could be prepared on a list in advance and crossed off as they come up. They'd be able to vary a bit in power, sometimes being minor but other times huge, but they'd always be helpful.
- 1 side would be marked with a Jolly Roger - "Ill Luck" - Something unexpected and unfortunate happens to the character. Again the GM could improvise, but I'd probably come up with a list like my "101 Wild Die 1's" chart from back in the day to fall back on if nothing inspired me during a scene. Players would know that when this symbol came up, things were going to get nasty. 5% seems about right.
- 2 sides marked with a Tower - "Situational / Location-based Event" - These would trigger a specific result depending on the location or scene. It might be good or bad, but it would be consistent through-out the scene. Examples: If the scene is a fight on a narrow ledge, rolling this symbol means you're falling off after this action. If the scene is taking place in the Imperial Palace, this roll means that your action is observed by a random powerful NPC courtier (and thus it's good or bad depending on who saw it). The GM would prep one of these for every scene they intended to run. I might do this symbol in two colors, so that it's possible to have two different effects, or severity levels, in a scene if the GM desires.
- 1 side would be marked with a stick figure, or a trio of stick figures - "Minor NPC Event" - this would mean that something unusual happens to some NPC in the scene, typically a minor one. This would again be something the GM improvises, but it would probably be guided by whether or not the task failed. If you fail, this symbol means some NPC working against you gets a lucky break. If you succeed, this symbol often means your action affects an extra target, or that an allied NPC gets a lucky break. This way we get around having to roll dice for all the extras, but also avoid the "you can't gain Stress because I'm sneaking up on you" problem with opposed rolls in Warhammer. If the PCs are alone in the scene, this symbol causes an NPC to arrive at the location (or triggers a wandering monster encounter if it's that sort of campaign).
Another option, though this would require more work (at least the first time) is to make small decks of cards that correspond to these symbols. Whenever the symbol comes up, you flip over the top card and apply it if it makes any sense in that situation. The main benefit of such a further randomization is that it would let the GM make some of the results really potent without the players feeling like the GM had it out for them. If made up on the fly or chosen off a list, there's the danger of favoritism or perceived favoritism, neither of which is fun.