WB are forging ahead with a Green Lantern movie, which is hardly going to be in the vein of The Dark Knight. I think they have realised that there is room for the gritty superhero movie (TDK being the obvious and best example) but also for the light-hearted one, set in the none-too-real world.
You know with the Green Lantern movie I have to wonder if their going to try follow Star Trek's lead and make a kickass space adventure due to getting their asses kicked by Trek in the boxoffice, like DC saying see look we have a movie about an interstellar peacekeeping organization and a bad guy who blows up planets, we're cool like Trek.
Look at how the movies and story were structured. Singer had a very clear two-sided conflict going on with the Mutants: Magneto's Militants and Xavier's more Peaceful advocations of co-existence. The Hellfire Club are a third faction in this, and their introduction would have messed with what Singer had been going for. Thus if he HAD included them he'd either have sucked everything interesting out of them and maybe make them just random HUMAN businessmen, or he'd just make them Magneto's lackeys. Anything else wouldn't work with his "Only Xavier and Magneto factions" thing.
Common sense, from what he'd done previously.
Given those possibilities, the Hellfire Club would be better off NOT being in his 3rd movie which would make Dark Phoenix sucky as well.
Heck, he'd have made Apocalypse into Magneto's lackey if they'd brought HIM into the movies...
Or they could have the Hellfire club remove Magneto and Xavier from the board so to speak like Sinister did in season 2 of the fox animated show if they want to focus on different characters.