I'd rather see Sinister in his own film. He more than pulled his own weight in Messiah Complex, Mutant Massacre and Inferno.
I'd also like to see Cyclops face down Sinister at the end of the film and utter the words, "get the hell away from my son, you son of a b****!" before completely pwning him. A good Cyclops/Cable dynamic is a must before Apocalypse hits.
After Apocalypse, the best places to go are Bastion, Donald Pierce and Proteus.