Transplant System[suggestion]

As time moves on, someone is bound to want more than one Kekkei Genkai or get their hands on some eyes that aren't particularly theirs. As seen in canon it is very possible to complete this action, but also should be implied the one losing the eye shouldn't die. To obtain their transplanted Sharingan or in one persons case a Byakugan, they did have to also get their current eye removed. Meaning it is possible to survive without the eye if done surgically and not just gouged out. All in all, just because someone gets an eye removed they shouldn't just die, even if it does get gouged out actually. Many ancient hero's have had eyes removed in more gruesome ways.

But obtaining the eye doesn't mean you can use it efficiently. Kakashi has been known to go under great amounts of strain through use of his Sharingan, so I think the Doujutsu should cost 2x the amount of chakra. This would discourage people from Doujutsu sharing(Making an Uchiha and giving a Mangenkyo Sharigan to their pal). And would discourage targeting people with Doujutsu, just for their eyes, since it would cost so much chakra to just use one ability.