Originally Posted by Lorus
This, I think, falls under the umbrella of accepting that the films are to be stand alone as well as part of a shared universe. If Sharon is not required to be in Avengers 2 then she'll simply not be included. The audience will be able to accept that not every character who was in the previous films will be in Avengers 2 providing that film tells a complete story with the characters it does use. Much like the Avengers was not effected by the fact that War Machine wasn't in it and the film provided no explanation or reference to him. For Sharon, it's not even particularly difficult to explain her absence because SHIELD is a massive organisation and she could be busy with any number of assignments.
I think the mistake that you're making here is assuming that the emotional development of a character or the act of providing them with depth is linked inseparably from their general skill set, in this case, secret agent. Natasha and Sharon may have reasonably similar skills but that doesn't mean that they have to share one specific story/character arc. In the Avengers, both Natasha and Clint are called 'master assassins' by Tony but they have different roles in the film and serve the narrative in different ways. In terms of the practicality of action, both Natasha and Steve use acrobatic hand to hand combat and yet they still fill different roles in all the set pieces without feeling remotely interchangeable.
I think it's also worth mentioning that you say that the character does everything that falls under that character type, but Sharon and Natasha fall under different character archetypes, they just have similar skills.
I agree with you 100%. I would say that sometimes you can differentiate/develop two characters even better
when they are both in the same situation. Tony Stark and Bruce Wayne are both billionaire playboy philanthropists, but their personalities and backgrounds are so different that you can't say that Batman and Iron Man are at all the same character.
As for Natasha and Sharon, you have one Russian femme fatale trained as an assassin since childhood, and you have one American child of privilege inspired by her heroic aunt/grand-aunt's war stories to voluntarily join S.H.I.E.L.D. as an agent. Sure, they're both skilled spies, but the similarities end there--they're not at all the same character.