Doom is the main reason I want the FF to be brought into the MCU.
Also, introducing FF and their rogues gallery into the MCU is a fairly straight forward process, unlike with the X-Men. But that's a conversation for another time.
One of the reasons Doom is so dangerous. Unlike Stark he doesn't reject the notion that Mysticism is just that... not a new tech to be controlled, but a Force to be harnessed.
I just hope they make Doom an actual credible scary threat once he's in the MCU. A Vulture/Killmonger level threat, but one who does not get killed off.
Man I would love to see the average movie goer's reaction to Maestro.
I think doing a flash forward seeing older versions of the current heroes (Maestro, Old Man Thor etc) could make for an interesting MCU phase of movies. The Maestro in particular would blow the minds of casual movie goers.