Necromancing this thread because I think it has useful ideas and want to add some of mine.
For covenant specializations:
-Neo-Mercurian theurges: Every fellow Mercurian has a few Communion spells and the group can work together to make their spirit pacts on the equinoxes and solstices. And then they summon daimons for a variety of benefits, using Mercurian Magic to drop the vis cost and Wizard's Communion to summon daimons in one spell. (Don't botch.)
-A contracting house: A covenant for Mercurian magi who have a variety of useful rituals for sale to other covenants, earning their vis on the margin.
-Cult of Heroes hero-crafters: A covenant specializing in increasing Characteristics. Caution: the covenant's magi are likely to have suspiciously similar statblocks after a while.
-Bookwriters: See above, but have the covenant's finest specialists boosted to +5 Communication (if they're fortunate enough to have Good Teacher, this is even sweeter).
-Specialist wealth creators: Ha, ha, ha. We are all very amused.
Plot hooks:
-Mercurian covenants are almost certain to contain a Flambeau or two (since there are a lot more Flambeau Mercurians than, say, magi Mercere). Quite possibly a Guernicus Terrae-magus as well. And they can Communion up either really high-magnitude destructive rituals (the Harbingers of Doom option), or else reduce the level on an Arcane Range Clenching Grasp of the Crushed Heart so low that the covenant's Protector can punch straight through an Aegis. A Mercurian covenant known to cooperate in Wizard's War will be feared, and their enemies will plot against them subtly rather than risk facing their anger in open war. Conversely, a Tribunal may come to rely on this covenant as the sword of justice against Marched wizards and threatening hedgies. And does someone in the covenant tend to pick up Arcane Connections wherever he goes? Vewwy suspicious even if it's legal...
-If the covenant grows larger, will it stay purely Mercurian? Anyone can learn a communion or theurgy, and very few rituals require more than a core circle of five or six Mercurians. If the covenant accepts more traditional Hermetic magi (either to perform specific tasks on the covenant's behalf or in general), will the covenant be able to maintain its Mercurian identity? If not, will the Mercurian circle remain a distinct group within the covenant? (The latter is likely - someone has to cast a big honkin' Aegis and the benefits of Mercurian cooperation don't go away.)
-The second generation of this covenant is going to have a reputation from the very beginning for a specific kind of magic, and many of their advantages (Mercurian Magic and likely a Focus - and this is assuming no Neo-Mercurians, and that nobody's going to awaken an apprentice to super-intelligence). To what extent will they continue the Mercurian tradition? Will they remain at the covenant, start their own all-Mercurian covenants or join other magi? If these magi remain in Mercurian covenants, particularly if they're pagans, then within the lifetimes of the founders we'll see a dozen or more magi raised in an insular tradition who may see their magic as not entirely Hermetic, and who other Hermetic magi may view with suspicion for their strange ways and magic. If the apprentices circulate among the Order at large, what legacy will they bring with them from their home?