I really like Covens and want to explore the various ways to use them in the game, gain access to them as an NPC or player, and see how useful/powerful the actual Coven casting can be.
Access/entry into a Coven seems to be limited to actual Hags, Changelings who follow specific feat paths, Witches with the Coven Hex, and Sorcerers with the Accursed Bloodline. I know there is a magical iron collar that allows participation, but I don't know how useful it is for this discussion.
VMC Witch to get the Coven Hex is the most useful way to get other classes into the Coven game, that I have found. And it is worth noting that VMC Sorcerer for the Accursed Bloodline does not work because the Bloodline Arcana is not part of VMC.
But once you are in the Coven... now what? The default list of spells available for Coven casting is uninspiring... some Hags and Witchfires and $#!+ add new spells, but what is the real power to this? Does being in a Coven provide any sort of bonus for ritual casting?
Would it be worth it to include Order of the Staff Cavaliers? Or worth it to invest in the Magic Domain's variant channeling? What about the Teamwork feats like Allied Spellcaster, Spell Chain, and Special Delivery?
Any way to effectively incorporate Life Link or Soultheif Vitalist collective into a Coven?
I'm not sure of the build details, but Dave the Commoner did some insane things with coven casting. IIRC, Caster Level went up into the hundreds.
Vitalists can only have a number of targets in their collective equal to their Wis Mod or 1/2 their level whichever is higher, and Covens have a minimum of 3 hags/witches, but typically max out at 13 hags/witches. You could take Expanded Collective feat to add 2 more, and take that feat multiple times (it's effects stack).
Also, I would get Iron Collar of the Unbound Coven so you don't have to put up with any evil hags in your coven. They're a pain to deal with (even if you're Evil aligned as well).
Do things that affect/modify the bonus given/gained by Aid Another also apply to the Caster Level bonus Aid Another grants in Coven casting?
I have a plethora of materials and builds in various degrees of completion based solely on or around Aid Another... if any of that applies to Coven casting, things could get wierd in a hurry... like Mudfoot was saying... CL in the 100's...
As for dealing with Hags, you just have to find the right one. Track down a Dreamthief Hag Souldrinker, strike a bargain, join her club...
Your more powerful Hags are infinitely easier to deal with. Although, the collar does bypass the need entirely.
And a 10-man Coven could be covered by a single Vitalist... that is still (or very well could be) a force to be taken seriously.
Some sort of mass-healing effect is necessary given the existence of things like the Pactseeker's Blade. If they can damage everyone in the Coven simply for being in the same Coven, we need a way to heal our community with equal or greater efficiency.