I've been working on this core mechanic for some time, and I think I need a few opinions from someone who hasn't been working with it to answer some really basic questions. This core mechanic is intended for an aggressively experimental strategy-horror RPG, and is intended to provide experienced players with power-user features.
The Composite Pool
Your skills and attributes are measured in die sizes, with D4 being the best and D20 being the worst. When you attempt an action, you will fill four die slots with dice representing relevant skills and attributes and roll them. Your GM may write custom Check Splicing rules to dictate how you can fill these die slots. Generally, you need a minimum of two skill dice to use a skill, and you may not roll four dice all representing the same skill or attribute.
Count the number of dice which roll 3 or lower as a success.
A number of mechanics--such as receiving assistance or spending extra AP in combat--give you Boosts, which allows you to bank your successes and reroll some of the dice. When you add a Boost to a roll, you may choose which dice to reroll, but you should roll all your Boosts out at the same time, and may only Boost any given die once. Again, count how many dice roll 3 or lower as a success.
Generally, the GM will compare the total number of successes you rolled against a set TN: 1 success is Easy, 2 is Normal, 3 is Hard.
A Few Designer Notes
The point of this mechanic is that there is no set "right" way to assemble a composite pool, and that optimizing the pool both encourages you to think in-character and rewards you with a better chance of success. It also captures a lot of nuance; in most systems, if you want to cook an egg, you have to roll using a dedicated cooking skill. In this one, you can create different die rolls to represent different ways to cook the egg. If you're hard-boiling it, you could go with your intelligence mixed with one knowledge die, and if you're scrambling it, you could mix dexterity and strength.
The reroll round also adds an element of diminishing returns to the roll. The dice in the pool are probably not all the same size, so if you reroll the smalles (best) dice first, you wind up asking yourself if it's worth spending extra to add in the larger dice.
The advantage is that the player gets these design features while having to do essentially zero arithmetic, at least to run the core mechanic itself. There are three tradeoffs. It can cause analysis paralysis. It requires a lot of dice fishing to set the roll up. Experienced players who know how to assemble the pool in their favor have PCs who almost never fail at tasks, provided the PC is good at what he or she is doing, and the player is having the character spending a reasonable amount of effort on it.
Well, two and a half. I think it's arguable the last one is more a positive feature that most GMs are not used to than an actual drawback.
The Questions:
Is four die slots and a default TN of 2 successes the correct way to go?
The system I intend to pair this with has 4 attributes and 20 skills. This means that with 4 die slots there is a little less than 1.6 million possible pool and Boost combinations (24^4 * 5). That might be a bit much. However, if I drop the number of die slots to 3, there's a fair bit less fishing for dice, and there's still about 50,000 possible pool and Boost combinations. But then I also have to drop the base TN down to 1, so there's no longer any way for the GM to make a roll easy. There's just normal, hard, and harder.
I've been messing with both approaches for some time, and when I play the game now, I feel like the three die slot version is claustrophobic. It isn't just that the difficulty granularity decreases; as a player, I don't quite have the creative space to use two skills at once or throw in a weird die to represent doing something strange, like throwing intelligence into a gunshot to lead a moving target. The four die slot version does that just fine. But at the same time, I've been messing with this mechanic for several years, now, and I am now probably far more comfortable with it than most players and GMs will be.
I know that three step die pool systems can work because that's how Cortex works. I've never seen a successful commercial system with four mixed step dice. But at the same time, this is a zero arithmetic core mechanic. Perhaps I can get away with it? I don't know. I'd really like some feedback on this point.
Would you prefer a three die slot version with better speed and practicality, but very little granularity and less space for customizing the roll, or a four die slot which adds the complexity of another step die, but has a lot of space and somewhat better granularity?