Shadow King! YES! Danger - YES!
Omega Red belongs in the next solo Wolverine film.
I totally forgot about the rest of those. Good call.
Bingo. I think throwing the HFC in XFC was a bit of a lost opportunity. I spent a lot of that movie thinking about how it would've made more sense and been much better if the main villain role had been occupied by Sinister.
And the FOH would make for a great minor sub-plot. I'm honestly surprised given how hard Singer rode the "X-Men as gay metaphor" agenda in his films that they NEVER popped up. Even in a quick cameo.
It doesn't matter if the timelines lineup in these films. XOW and XTLS don't synch up. XFC doesn't sink up with the trilogy at all. We're told that Cyclops, Jean and Storm are some of Xaiver's first students in X-Men. Yet in XFC, Storm is a quick cameo and everyone else is nowhere to be found. But they included the other Angel and Darwin.
Back on topic, I am actually pretty glad that the Sentinels are finally being brought into the franchise. Even if I did wish that they had given them a revamped look. Namely made them smaller.
So far the pictures make them look like the toys they're going to sell based on the movie. But I'm holding off on final judgment until some footage of them in action comes along.
I don't think the Hellfire Club was a wasted opportunity at all. I think the Hellfire Club was a great choice to have as a villain for an X-Men film, and I believe that X-Men: First Class was great timing to do that.
As for your comments on the timeline:
I don't see how X-Men Origins: Wolverine and X-Men: The Last Stand don't sync up.


They don't particularly reference each other, because on the timeline they are so far apart from one another, but I don't see how they don't sync up.
X-Men: First Class absolutely does sync up with the rest of the trilogy. Xavier said that those people were -some- of his first students, not that they were his first students. Introducing them as his next line of X-Men would absolutely sync up. Especially when Xavier would have tons of students AFTER Cyclops, Jean, and Storm all came to the school, such as Iceman, Rogue, Kitty Pryde, Siryn, Jubilee, Artie, Jones, Colossus, just to name a few. We also have Beast as a member of the first class, who was established in X-Men: The Last Stand to have been a former team member who left and is now coming back. That syncs up. Also, keep in mind, the school doesn't actually open until the end of X-Men: First Class, and at that point, Xavier's only remaining students are Havok, Banshee, and Beast. The school isn't even officially open at that point, and considering that X-Men: Days Of Future Past shows Xavier as an emotional wreck 10 years after the fact, it could be very possible that the school never even opened at THAT point. So... it could be assumed that the events of X-Men: Days Of Future Past get Xavier back on track to open the school, and with Banshee known to be gone (as he's not even in the film), and Havok seemingly leaving during the events of the film, and Beast leaving at some point as established by X-Men: The Last Stand, Xavier would need to recruit a new X-Men team, and his "first students" in Cyclops, Jean Grey, Storm, etc. The opening shots of the movie, and Magneto's entire story arc tie into Magneto's sub plot in the first X-Men. Outside of a dialogue inconsistency with Xavier saying he was 17 when he and Magneto met, the story arc of X-Men: First Class matches up perfectly with Xavier's description of their history.
The timeline isn't crisp and tight, for certain, but at the same time, I think it's fanboys who study these films WAY too much who are the ones looking at making sure everything lines up perfectly with specific dates and ages. The timeline problems in the X-Men series are no worse than the timeline problems and continuity contradictions of the Star Wars movies.
As far as the Sentinels go, I think they look great and I couldn't have expected them to look better than they do. I think this is an unfortunate case of "damned if you do, damned if you don't". I think making Sentinels smaller would be a horrible choice. And I'm sure there are many others that think the same way. But make them big and purple - I.E. comic book accurate - and now the criticism of looking like toys comes in. So basically, Singer stays comic book accurate, he's accused of being too comic booky and making something that looks like a toy. He strays from comic book accurate, and he'd be accused of needlessly changing Sentinels and making them less menacing.
I applaud Singer for this one.