The Taskmaster is arguably one of Marvel's greatest villains to date. The problem is, the film rights for him are currently owned by 20th Century Fox, and if Fantastic Four is any indication, he's not likely to return to the Marvel fold any time soon. That being said, if Marvel were to work out a deal with Fox, much as they did for Spider-Man with Sony, I think The Taskmaster would be a pretty incredible addition to the existing MCU.
There are a number of reasons I feel that The Taskmaster should rejoin the MCU, not the least of which is the fact that not too many people really understand how he wound up becoming a Fox property in the first place. I would elaborate on how that happened, but I don't understand it either. Anyway, I think it's pretty safe to say at this point that unless there's a major change in the wind, we're not likely to see The Taskmaster in a film that has the Marvel logo any time in the near future. Another big reason I want to see him rejoin the MCU is the fact that, although he is a mercenary by profession, he frequently has ties to a certain terrorist organization that MCU fans the world over know and loathe: HYDRA. As big as these reasons are, however, they simply cannot compare with the main reason that The Taskmaster should rejoin Marvel. That reason is, he's just generally a great character and I seriously fear that Fox will not give him the justice that he so richly deserves.
Now, for those of you that are unfamiliar with just who the Taskmaster is and what he does, let me give you a brief rundown of the character. Born in Brooklyn, New York, Tony Masters demonstrated a very unusual ability during childhood. After watching a cowboy film on television, young Tony was able to instantly replicate the intricate rope tricks performed by the cowboy. His mother was quite nervous about this ability and summoned several psychiatrists, who after examining Tony, determined that he possessed a form of photographic memory which they termed "photographic reflexes". In high school, Tony used this ability to become a star quarterback after watching only one pro football game. Following graduation, Tony briefly considered using his ability to fight crime, but determined that engaging in it would be much more lucrative.
After watching archival footage of several superheroes and villains, he carried out several successful grand larcenies. However, when unforeseen dangers arose, Tony decided to instead open a facility to turn amateur criminals into dangerous professionals.
Tony eventually created a costume and took the name "The Taskmaster", and opened criminal academies all over the United States. However, one of his academy supervisors cloned himself to use as an organ donor using the school's technology. The clone however, realized what was to become of him and contacted the Avengers, who sent Ant-Man, Wasp, and Yellowjacket, all of whom were summarily captured by The Taskmaster. The rest of the Avengers arrived to rescue them and The Taskmaster barely escaped after battling Iron Man and Captain America at the same time.
The Taskmaster managed to escape, but only after an attack by Jocasta, whom he was unable to defeat because the robot's lack of body language hindered his ability to imitate and predict its motions.
The Taskmaster ultimately was betrayed by Red Skull and taken into custody by the government, but Red Skull eventually had a change of heart and arranged for his escape, allowing him to return to his activities as a teacher of criminals.
The Taskmaster's abilities are many and varied. They include extraordinary fencing/swordfighting prowess, being an exceptional martial artist, a nearly-always accurate marksman, voice mimicry, and what are referred to in the comics as "photographic reflexes", which is a fancy way of saying that he can mimic the fighting style of anyone he has ever observed in combat or personally fought.