So, this idea is going to require some suspension of disbelief, as I'm going to be suggesting something really OP. I will also give a less OP idea, but I'm excited about the big one. If you read the title, I'm sure you will have some inkling of what I'm thinking of. This will be separated into two different sections.
First is the really OP idea, where the MC would have access to most of the different Dragon Slayer Magics. I'm aware that Black Clover's magic only allows one attribute per person, but we're talking about a foreign power system. I'll list the possible magics in the next paragraph, but let's just say that the MC would be able to use all the ones I list there. I'll also be selecting ones that aren't canon but they are reasonable choices.
The second is more like giving the MC normal magic, just super-charging it with the ability of becoming immune to and eating that respective element. Of course, having one attribute doesn't mean only one magic, as elements can be broken down into different magics i.e. Wind and Lightning and Water and Ice. So I'll be suggesting possible Slayer Magics for the protagonist. Sky Dragon, Fire Dragon, Water Dragon, Ice Dragon, Iron Dragon, Lightning Dragon, Shadow Dragon, Light Dragon, Cavern Dragon, and Poison Dragon. Both the Sky and Water magics have healing and buffing abilities while having strong offensive capabilities, as any other Dragon Slayer Magic does.
So, I'm thinking as a way to explain the multiple magic attributes for the OP MC, is some special variable Mana. Maybe say something like Dragon Mana or Dragon Magic, it's better than completely glossing over it. Either that or use a "wish" to cause people to ignore this special constitution the MC would have. Neither of these are the best solutions, but I can't figure out any other possible explanation for the OP version of the protagonist.
As you could probably tell, this was more geared towards reincarnation, but you could just as easily spin it for an insert character. Although, that would only work with the single element idea. I really just wanted to see how strong a character could get in Black Clover.