So my current idea for this is something I think of as "Master and Apprentice". Basically, every player makes an apprentice character, and then comes up with a concept for that apprentice's master. The master is in the same order as the apprentice, a power player in mage society, and he or she has an agenda. The characters you would play regularly would be the apprentices, but you might pull out the masters if things get too hairy.
Here are some details:
Players set their masters' agendas during downtime when appropriate, and the apprentices are the ones who have to do the street-level work for the masters while they do more important things. Some reasonable agendas are: research the Abyss, investigate the Seers of the Throne, forge an alliance with some other supernatural group, gain political power among mages or other supernatural beings, gain temporal power (e.g. wealth), claim a Hallow, misdirect a sleeper cult to prevent it from harming another mage, etc. Agendas cannot be things that specifically benefit the apprentice, but what helps the master might also help the apprentice (for instance, when a master gain a consilium seat, it will help the apprentice if he gets to be the master's provost).
During play, I would control the masters based on concept and history, and they would stay out of the story for the most part. I think that this all would work best if the masters had their own cabal, with the players' characters being subordinate members of it. If everyone agrees to that, the cabal will have an overall agenda as well, one that helps most of the masters achieve their goals. The default agenda for this is that the masters want to set up and maintain a "neutral" establishment where supernaturals can come to do whatever it is they want to do (within reason). The masters have chosen to operate an exclusive nightclub, its reputation enhanced through subtle magic. The player characters get to be the grunts that smooth over any bizarre crap that happens, while occasionally performing tasks for supernaturals who hire the cabal to take care of things.
As far as stats go, the masters each start with ~200 experience points. They must have at least 3 dots in Gnosis and 5 dots in an Arcanum. They can buy artifacts and imbued items with this experience, or spend 8 experience to buy any magic item that their arcana knowledge would allow them to make (within reason: thaumium would require a good bit of cash, for instance). Legacies cost 10 experience per attainment for the masters. A master can make a Demense at the cost of 1 Wisdom and 8 experience. If you don't want to stat one out, all I really need is a concept, an agenda, and 2-3 sentences of back story. If you have old Mage characters that you want to use again, this would be a good role for them to take.
The apprentices would get no bonus starting experience, and each start with Gnosis 1, at least Mentor 1, and at least Resources 1. Dots in Mentor determine how much the master likes the apprentice and how much the master is likely to interfere with the apprentice. Masters will not generally share their Mana or magic items with their apprentices unless it is an emergency, but masters may have Demenses and they may be willing to give apprentices access (as a guideline: free access at Mentor 4, provisional access at Mentor 2, but dots won't matter if you have a good enough reason). Since artifacts gained from the Artifact merit produce Mana like a Hallow while giving other benefits, I think that they make a good alternative investment.
