My group has decided to take a break from the "punch-in-the-gut emo porn" we've been playing lately. Because we are insane, our next game (starting in early July) will be in the Spelljammer universe, using the Full Light, Full Steam ruleset, a slightly reinterpreted as "fantasy steampunk anime." I've got a reasonable idea of how to handle the setting, but I know the rules will require a few tweaks. Specifically, I'm not sure how I want to handle the following:
Magic
Spelljamming
Ship-to-ship combat
Thoughts? (Josh, I'm looking in your direction, but anyone can feel free to jump it.)
The place I would start would be Attributes and Skills. Attributes you might leave as-is (if you're okay with a strong emphasis on Leadership) -- look them over and determine if they feel 'right.' Skills you'll probably need to rip out and replace. Pick three broad categories of action you want to see in the game (Heroics might stay; Engineering could be replaced with Magic, maybe; how important is Culture in the setting? What do characters do in the setting?). Then find ten specific things that fit into each category.
Note, tangentially, that races make excellent thematic batteries.
My first thought is to treat Magic as Engineering (is there any mundane engineering in the setting?). Look specifically at Gadgeteer. The skill lets you do anything with an appropriate little gadget. Said gadget need not be prepared, built, or bought beforehand; you win the roll, you have an appropriate gadget. You could do the same with spells under categories like Healing, Pyrotechnics, Wayfinding, Scrying, and so on. Make the roll, your understanding of that kind of magic is adequate, and you can fill in the color details of the spell's name and whatnot after the fact.
Ship-to-ship combat is easy to handle, actually. It's just a cooperative roll with most-or-all of the players acting as crew under the commander. Losses result in Hull condition battery discharges.
So in the original Spelljammer, most ships are powered by helms, which, in essence, suck the spellcasting power out of the person sitting in them. Sitting in a helm removes the ability to cast spells that day. The more powerful the spellcaster in the helm, the faster and more maneuverable the ship is.
(To jump back to the earlier comments, it is mostly Skills that I'm looking at replacing.)
Oh, that reminds me: you'd need to consider whether the Health/Grace/Will condition batteries work for Spelljammer. For instance, spelljamming for a day could just discharge a level or two of Will. (Or have the spelljammer make a static check against some high difficulty (even 13) and use the margin of failure to tell you how many levels of Will they lose for spelljamming that day.)
Does Spelljammer have a core story or anything? I think Heroics and Magic would probably do for two skill categories, but I don't know the setting/game well enough to speculate on a third.
I like the idea of magic being treated like gadeteering. I would also treat it like as a characters condition battery. What I mean is sort of a combined skill and condition battery. Simple, low difficulty, spells could be done without discharging the battery while the difficulty of more complex spells would likely only be achievable if the character discharged his magic battery. The actual battery level wouldn't decrease until after the spell roll is resolved.
For example, I want my character to be a mage so I spend some of my points to have a magic battery of three and an acuity of three. Now I want to cast some really powerful spell, like summon popcorn, and we decide it has a difficulty of 12. I will need to be able to count a six twice so I decide to discharge my magic battery and my reckless spell caster thematic battery each for one point. I now have a four and a four, since I don't actually lower my magic battery until after I roll. I roll a one, three, three and a six so I succeeded and now have a nice bowl of popcorn to munch on, but my magic battery is now at a two.
I would treat the helm as a ship's condition battery. Sitting in the helm springs a characters magic battery but charges the ships helm up to the level of the characters current magic battery.
Does that sound workable?
Edit: I would also say if you want to boost your magic you must discharge you magic battery. Depending on how much you want to make magic be a limited recourse, I would say at least one point of the boost to magic must come from your magic battery or at least one. But, you could still boost your stat by discharging a thematic battery.