There is a God Eater collaboration weapon set for the game and the unknown origin of the Thorns of Judgement can easily be the origin of a rogue Aragami boss. It can also be used to tease at the upcoming God Eater 3 as well.
- Jossed. That being said, Dyaus Pita does cameo in a cutscene revealing that Code Vein is a God Eater spin-off.
- Confirmed. The DLC's Hellfire Knight, Frozen Empress, and the Lord of Thunder are a Hannibal, a Marduk, and a Vajra Aragami respectively.
We know that we'll uncover the Protagonist's memories during the story, as well as probably Jack's. Given how Word of God has said that they had a deep connection, it wouldn't be surprising that their relationship started when they where humans, before losing their memories.
- Mostly jossed, given that their first meeting was during Operation Queenslayer, but there is still the possibility that they briefly met as humans but they don't recall (since being a Revenant also means losing memories each time you die).
The Protagonist is shown to be able to drink Cruz's blood without becoming a lost, which could be explained by them being related, though we don't know if Gregorio is affected by his daughter or not.
- Jossed. The protagonist is one of Cruz's "Successors" due to dealing the final blow during Operation Queenslayer.
I am making this guess based on some footage where she seems to be caressing the protagonist, and I am guessing that this is a hint that she did something to him/her that caused them to be able to survive being shot in the heart as well as revitalizing the tear springs. Maybe she did this to further her own agenda.
- Confirmed. She made them one of her Successors.
We know the Protagonist got killed two days before the story began (which might be the moment they became a Revenant), and since we know their past is gonna be uncovered, it's likely that something sinister is residing in those lost memories.
- Unfortunately jossed.
- Jossed. The Successor of the Heart is Karen meanwhile your character is the Successor of the Blood.
Or an alternate timeline where researching Oracle Cells led to the creation of Revenants. We'll see.
- Jossed. Theyre in the same universe.
Some time after God Eater 3, God Eaters either start dying off or become obsolete. To replace them, oracle cells are modified to produce Revenants. While this was mostly successful against the Aragami, being a Revenant causes unfortunate side effects such as needing to consume human blood to prevent going into a frenzy, thus pushing scientists to create the Queen, leading to the events we see now.
Given the prominence of Ash in what the Revenants turn to whenever they die, and especially if they die for real, it's entirely plausible that the whole reason the Ashlands and the Ash Aragami that turned Earth over on its head even worse than before may have been due to either a freak accident or a pre-Gaol of the Mist incident of Revenants being genuinely devoured by Aragami and accelerating their evolution on a dangerous new path. Given that the last bit of content for God Eater 3 involved Claire's Episode, and particularly Soma/Ein revealing he had six months left to live, it could be entirely plausible that Code Vein might be setting up a crossover or intervention in post-GE3 plot as a result.
Juzo Mido makes reference to the fact that they were able to deal with the horrors in their isolated area of the world, horrors being used primarily to refer to Aragami. And only an Aragami can hurt another Aragami. So its entirely possible that the Revenants are capable of battling Aragami due to being Aragami themselves; or more specifically, the BOR parasite is itself an Aragami that relies on a host to feed itself. This would explain the craving for blood, and also why Revenants mutate when they lose their humanity, or frenzy; they lose the one thing separating them from the monsters they fight. After all, humans have become Aragami before, havent they?
The events of God Eater 1 turn the moon green, and since the moon isn't green in Vein, it's safe to assume that it predates those events, which happen at 2071. Meanwhile, side material for God Eater 1 say that there weren't any Aragami bigger than Kongous before 2065. That narrows down the events of the game from around that time frame. It may even be possible that the events in Vein happen around the same time as God Eater 1, as the region of North America hasn't really been explored or talked about in detail.