Adjustments to my homebrew Risus Fantasy rules for the spelljammer game are as follows:
Don’t worry about picking a power source for any magic-user characters. All magic is pact-magic. Clerical, wizard, sorcerer, whatever. To get magic you have to bind yourself to a supernatural entity of some sort. An angel or demon or alien power. All magical power has adverse affects on mortal flesh in prolonged use, so mutation rules are still in effect. Everything you can sweat a pact with is pretty much trapped in a single crystal sphere and uses those pact-sworn servants to spread it’s influence to other spheres. Most entities can perceive everything their servants see/hear/touch. The rituals to enter a pact work from anywhere, so many servants try to convert as many others to their ‘cause’ as possible. Each entity can also grow in power, but the precise form of that feeding it that power differs with each one. Angels receive prayers of the faithful and grow strong. Demons are similar, but gain strength from sins committed in their name (such as ‘the demon made me do it’). Some alien beasts eat souls, others may need starfire channeled through spells back to them. Every magic-user should come up with the ritual or act of devotion that makes their patron stronger in some small way.