Okay, so Mage's Association's probably gonna have their fingers in this whole situation, as well as the Holy Church. I got an idea that maybe some of the Masters have been recruited by the Association as soon as the word about the 6th Grail War got out 12 years ago. Those Masters would be in direct contact with the Association and each other throughout the war, serving their cause since the Association would like nothing more than getting their hands on the Grail at this point. They wouldn't do it for free, oh nonono. The Association would grant particular resources to those Masters, like money, place to stay, certain magical thingies
That would split the Masters into two groups. Those with the MA and the lone riders. The first objective of the MA mages would be to eliminate the others as quickly as possible to ensure victory for the Association. It would involve teaming up against other Masters just to ensure the kill, which would probably make the other masters team up as well in order to survive.
Also, don't think the MA guys are some mindless MA drones. If you choose that option, your characters would still probably seek the Grail for their own benefit while simply exploiting the offerings of the MA. Considering that MA is stubborn, please do prepare yourself because they aren't going to let you just walk away with the Grail after spending their funds. They want the Grail for themselves and to find out exactly why did it reactivate and how, don't forget.
Maria falls under this group, but only because she herself is weak (because of reasons currently unknown), although she keeps her distance from the other members of the MA group. She gets what she needs from the Association even before the war even began, so she isn't going to stay with them if she doesn't need to. Sure, she is now one of the hunted ones, but she doesn't care. Her Rider can get her out of a pinch.
About the Church... It's just there, nothing special. A new priest has been sent to the now empty Kotomine church in Fuyuki so the Church still serves as a safe place for Servant-less Masters.
There's also the question regarding the 3 founding families, Tohsaka, Einzbern and Matou/Makiri. The Grail isn't picky this time though. It doesn't have time or resources currently to throw around just so it could get the 3 families involved in the war. However, if any of you really want your PCs to be a part of one of the families, feel free to do so, but pay attention to the genetics and family stuff when figuring out your character. Do some resarch about them ~
(and yes, you can say your PC is a child of one of the already established Fate serioes characters like Sakura Matou or Rin, though I won't accept the Rin proposal cuz I have plans for her)
Post your complaints about this in the comments below ~ I'm off to sleep.
Cya