Internal factions within Nazi Germany striving for hold on power?
Muscle of arms/Army
Magick
Daemons/Angels/Religious extremists
Eugenics/Bio-engineering science
(At expense of overall Nazi power or just for control within nazi power?)
Abwehr
Wehrmacht

Where does Hitler fit in the grand scheme now?

What extra elements do Japanese offer that Germans cannot access alone?
Dragon cults
Alternate Magick
Hi-tech weaponry/mech development?

Resistance factions?
British
French
Soviet/Russian
China
Other individual nation resistance
Combined anti-nazi

Teams that were set up during rise of new Nazi powers that are still operational despite fall of nations. These could be focus groups assembled to counter specific Nazi threats, eg magick / occult / religious / etc..

Problems Magick users may face:
There is always the idea that magick is powerful but dangerous to those who wield it, (like a gobbo shaman eatin too many magic mushrooms)
Since you’ve been carefully unspecific as to the nature of magick it could affect in many ways:
Disfigurement/Ageing/skin taint – eg Starwars emperor
Instability
Lack of control – ie user tapped into 1 form in particular and now he/she cannot control when that power is used (becomes random / occurs when user is asleep/distracted/annoyed)
Magick/Daemon host – magick takes over user completely
User becomes blackhole/portal to magick realm/source
Sensitive to light/other stimulae (could be relative to which type of magick they’ve been using)

Polish unknowns:
Lycanthropes / other were-creatures?
A natural magic that is tied to the land, the dark tide emanates from a source here, either spread across the wilder untouched places or cocentrated in 1 or a few spots where magick users can access more raw power than they could dream of, or even control.
The real reason why germans don’t trust the polish – they’re all latent magick users which gives them weak powers which they almost don’t realise they have. (it makes other nervous or itch when around them) They could have a better control or use of magick than other populations in europe and the secret shame as the germans realise that the only reason some of their number have developed talents in magick is due to cross-breeding with their enemy.
Germans feel the need to use concentration camps to kill the magick in the polish so that only they control it.
The land is sentient and will subtely attempt to resist invaders, through ‘natural’ phenomenon, sometimes subtle like timely/unexpected mists to make soldiers lost in woods and go rnd in circles or more obvious like landslides, localised earthquakes etc as GreenOsprey suggested.

Very intriguing scenario
Looking 4ward to more.

I like the idea of ‘The Keep’. A horror about Germans located in a old keep, with something hunting them – awoken in the depths. Always makes me think of a patrol stationed in the wilds, coming under attack by unnatural forces, defending the land against the evil Jackboot of the SS!