Adding the Mangekyou Sharingan isn’t easy because ,as we all know, in order to obtain it you need to kill your “best friend” or at least someone you deeply love in order to feel a great pain. If we want to add it we therefore need to add the concept of friendship in Naruto C.
The idea i have of the concept of frienship would be:
-You can have only one “best friend”.
-You are some kind of “reputation points” with this friend.
-Whenever he dies, you lose points, whenever he kills, you earn points. Very simple.
-If you kill him accidentaly (which happens during fights) or on purpose (which happens if you are a psychopath or an uchiha) you lose all (or at least a great amount) of your reputation points and you get Mangekyou if you are a uchiha and if your reputation points were at least positive(if you don’t really love each others then where is the pain?).
-If your reputation points are positive your stats get better (like *1.1, something like the majin buff in DBC). If it’s negative then it’s exactly the opposite (like *0.9).
So having a best friend is dangerous but worth it, especially if you are an uchiha. And it makes the Mangekyou hard to get, which is normal in my opinion. Mangekyou is only achieved through pain.