Is this the same book Harry finds to start his Harem? Is it like an Elder Scroll and is a floating hole in all realities or something? I was just going to have the witch to put a curse on the big house that was now a rental property that turn the staff and tenants into monsters and time lock the estate to spite a man who chose another bustier woman over her. As the story progresses, with each encounter the hero breaks a part of the curse and restores the women to their natural state of being (Mummy becomes an exiled aristocrat who lives in the Royal Suite, the ghost is the Mistress of the House, the werewolf is the Gamekeeper etc) and fixes that portion of the house like how sucking up ghosts in Luigi's Mansion illuminates and cleans their rooms. Culminating with the witch herself and then the 5 horror movie classic chicks, who are apparitions the curse made to spite the man, who was turned into the old, creepy, demented undying caretaker of the inn. Upon sating the manifestations of the curse, it is broken and the man regains his youth and vigour, thanks the kid and adds his name to the title of the estate so he can travel the world he's been gone from for decades and work on other ventures.
Now for this fic (and I am sorry if I am overwhelming you with my Marvel comic book nerdiness). I am leaning towards a simple rewrite to where instead of a transdimensional accident, they have a mere teleportation device accident that bamfs them into a battle zone between X-Men and Avengers against a yet to be decided villainous threat in Chicago (because destroying New York is getting redundant). They are all in the same universe except since the Big Hero 6 are relatively new and operate mainly on the Western Coast, this fight serves as their formal introduction to the greater superhero community.
Now the X-Men will be the Daughters of the Atom era since it's already all female, which means we'll have to add Psylocke. Jean Grey is currently deceased (although on a scale of 1 to 10 How Dead Is She? she's like a 3 and I am tempted to bring her back later on for some mother daughter time with Hiro- if you're into that whole incest thing. If not then ignore) and Rachael Grey holds the Marvel Girl moniker. Also Ms. Marvel Carol Denvers got promoted recently and now fights for the Avengers as Captain Marvel. The title of Ms. Marvel is now held by young adult super heroine Kamala Khan, whose body morphing power of 'Embiggening' actually provides an in universe explanation for fantastic bust size although I don't know of any middle eastern mega bust models to proportion after. Weather or not you want her in is up to you, she's kind of really new and I am still warming up to the character. Also the current director of S.H.I.E.L.D. in Nick Fury's absence is agent Maria Hill.
I'm also toying with the idea of swapping Steve Roger's Captain America with 'American Dream' Shannon Carter (which is essentially a female Captain America). Otherwise the Avengers team is the current comic book incarnation, Agent Venom, War Machine, Captain Marvel, Invisible Woman, Thor and Black Widow. Crystal, She Hulk and Valkyrie answered the all call and are present at the battle. Dazzler happened to be in concert in Chicago when the battle kicked off.
As for villains I'd like to see Mystique, Emma Frost, Enchantress, Polaris and Lady Deathstrike. More will be added if need be but know I'll want to play with those characters.