MCU doesn’t really have a ‘proper’ Reed Richards, so the alternate universe Evil Reed from Secret Wars couldn’t work that way. The only brains of the MCU that could fill that role in that plot would be either Stark or Bannon, and the latter is a) still alive and b) already his own foil and his genius isn’t really played in the same way anyway. B-list alternative would be Hank Pym but he’s not been central to the MCU in anything like the same way as the other two.
Honestly I think it might work pretty well story-wise. Though actual reason is just… well, money. And the course correction aspect previously mentioned in these comments.
Sure, and that will be great I’m sure, but an evil version of a character we’ve just been introduced to won’t hit the same.