for some reason I keep getting the felling that it will be Sentinial Lyons and some brotherhood hearing that the brotherhood in the west coast (or Mojave) has crumbled, which isnt a bad thing but not the thing I wanted.
Seems unlikely to me. The Brotherhood in DC has drifted so far from their original mission that they don't have much contact with the rest of the Brotherhood anymore. It is also possible to kill Sarah Lyons in the Broken Steel DLC, and I don't think the developers would let a non-essential character return. Dr Madison Li, however, can't be killed in any way as far as I know, as she disappears from the Capital Wasteland in Broken Steel, and is always essential before that. So we may see her return in a future Fallout game or DLC one day.
Doesn't Arcade Gannon's dark secret already get revealed in the Vanilla game?
I want to know what this place that 'players thought they would never see' is.
On the Arcade part; that's a mistake on the source's website, which makes me doubt pretty much all the other info on there. During Arcade's quest his past is indeed revealed.
I think that place, - if any of the information is the website is correct - , may be a FO1 or FO2 location. Maybe the Hub, or Shady Sands. Is a bit far away though, but who knows.