In the discussion of system mechanics, first off combat: I instituted a bit more of an initiative system than was recommended in the ‘whenever it makes sense people go’ structure of Buffy the Vampire Slayer – people with the Fast Reaction Time quality go first, then everybody else; inside of that we count down by Dexterity, and break ties by whether the action is guns, moving or punching. It felt like it would move quickly enough.
I then cut out a lot of the fiddly martial arts moves from BtVS since the expectations were different – people playing BtVS likely wanted that degree of granularity but I didn't see it for Girl Genius. I also had to reverse engineer the logic of the attack system (what gave a penalty to hit for more damage or different effect? How much damage to various weapons do? What doubles damage and what doesn't?) and figure out how to work in high energy weapons, which do about as much damage as fully automatic weapons. The Buffy Universe doesn't truck much with firearms since they don’t work well on Vamps, so I had some concerns there.
That being finished I had to tackle the Inventing rules. This meant bastardizing the BtVS Magic Rules and adding a lot of fiddly bits specific to both the needs of inventing, having multiple people working on a problem, how powerful Sparks can be and the nature of the Girl Genius universe to raise both tension and humor via interruption. My idea was that the biggest limiting factors were time and skill, and if the project takes too long the PCs will doubtless be interrupted and need to engage in side issues while trying to keep the original invention going. My hope is that this will keep the madcap nature of the source material in place and give ways for people not involved in the invention to have something to do.
This ended up being a bit of a mini game. That’s game design speak for a set of sub rules inside the main rules that are used when a particular action set comes into play. Early D&D was really just a set of overlapping mini-games (the rules for combat used different dice and mechanics than the ones for exploring, which had a subset of different rules for Thieves, and neither used the same mechanics social interaction). I figured anyone who wanted to play a Spark would be willing to go through the extra effort to learn and master the mini game, which at least had some of the same dice concepts as the rest of play.
One last things about the BtVS rules is that they’re completely player facing – the NPCs have a very small stat set (Muscle, Brains, Combat) and the players always roll against those. NPCs don’t make stealth rolls, the PCs make notice rolls. NPCs don’t make notice rolls, the PCs make stealth rolls. When I’ve run BtVS my palms end up itching to roll some dine, but I can survive.