I see your dilemma. I have also put my A!C campaign on temporary hiatus because of the very much anticipated A!C 2d20 release. We have been using FATE up until now. It´s great for storytelling but a bit too loose in other areas.
I was actually trying to cook up something homebrew style. But it´s just too much work and it is very hard to establish consensus among the players on how things actually should work. It tends to draw focus from what I want as a GM: a strong narrative driven WW2 counter intelligence setting with paranoia, occult and outright supernatural themes. I need a slightly pulpy ruleset that help bring this about.
You should totally read up on WW2, there’s a ton of relevant history and many fascinating biographies that make great inspiration for adventures. Since the official secrets act of 1989 opened the lid of the archives somewhat, a lot of SOE and “secret war” information has been released.
I personally think CoC/BRP is a bit unexciting, and FATE is too loose. I have no experience with Savage Worlds but maybe it´s worth a try if you are pressed for time and want to get going?
I will definitely update my WW2 knowledge–I don’t have nearly enough “on hand” in my brain, and you’re pointing out rightly that I ought to (for fun and for better GMing).
I’m cooking up 3 other options, as well, and I’d love feedback:
Run / play a session of modified THE QUIET YEAR to establish the players in a secret organization setting they care about.
Run a session of players as Terrible Nazikultist Monsters or Sorcerers where they establish the big bad of the upcoming campaign. This could be in any game system. It could even be something wild like a flashback to dark ages Europe where we see the elder gods planting a terror not to be awoken in 1,000 years.
Run a session in 2d20 “modern” with pregens where the players are the “last” team of Good Guys who Die Horribly and in whose absence the PCs now have to step up to the plate, launching the new campaign.