I'd like to see Shadow King and Sinister have cameos in Storm and Cyclops' intro scenes but nothing more. Shadow King would just be a random crime boss with psychic powers in Africa while Sinister would be shape shifted into another form and work at Cyclops' orphanage where Xavier adopts him.
I feel like Apocalypse and his Horsemen are enough for one film and I'd rather just tease these two. IMO, the two together should get less than 5 minutes of screen time. I'd have Sinister be the bad guy who the films build up to slowly and tie him in to the origins of Magneto, Cyclops, Gambit and Wolverine. Shadow King would just be a cameo who everyone should forget about by the time we see him again which would be a long, long way away.
Here's how I'd handle Sinister. In Apocalypse he gets a small role in another form during the first act. He then reappears after the credits in the Alkelai Lake Weapon X facility alongside Sabretooth.
I'd then name drop him in X-Force but have Genesis/Tolliver and the Dark Riders as the main villains and Stryfe as the larger threat in the background. I'd base the film around X-Force Vol 1 1-15 and Cable Vol 1 17-19.
Then come the next X-Men film, it would be all about Stryfe and Cyclops as an adaptation of X-Cutioner's Song. Sinister would have a small role to play but he'd just be in the background observing things aside from making a deal with Stryfe at the beginning of the film and begin the mysterious figure who saves Cyclops' life.
I'd use him as a villain in X-Force 2 which I'd have be the adaptation of New X-Men 142-145. I'd also have Malcolm Colcord and john sublime appear and both die with Sinister left in control of Weapon X with no rivals.
Finally, I'd do a medley of Inferno, Mutant Massacre and Messiah Complex where Sinister's plans come to fruition and we see just how big of a threat he is.
)?
