For Captain Marvel, some options:
Ghazi Rashid
Not the most interesting option, he was created solely to be powerful enough to take her on. He is just cruelty in a suit, so I'm not sure what else they could do with him...
Moonstone
Lot of grey area here. Still, she's the direct counter to Carol as far as 'super powered woman'. Might even do the 'stolen identity' story concept. Also might help lay some groundwork for potential Thunderbolts later down the line.
Cru
By far the most interesting, but would require so much groundwork to be laid. Would also potentially mean that the first step has Carol taking on the Brood, and I'm not sure if Marvel should go with 'swarms of enemies' again. Still, there is a lot they could do with Cru.
Whoever they go with, they really need to show Carol's power. They've already stated that she's going to be the most powerful hero in the MCU, but I doubt they'll give her Binary level powers.