SoftCrow
Yo author of one of the eye abilities is like sage mode. Then he should have all possible advantages of sage mode and easier path to learn sage mode in future. I mean mangekyu always manifests as users inner desire, and their area of expertise. Also there should be some other features to it. It feels very underwhelming compared to some others. Idk if you would apply the idea but maybe the ability on further evolution would provide chakra cloak and help in higher level of technique like instead of red/orange fire ball he can launch blue/purple fire ball which is hotter and faster.
The Mangekyo is so rare that very few Uchihas have activated it over the years. There haven’t been enough members to test whether the eye transfer works between any members of the Uchiha clan to obtain the enternal Mangekyo sharingan. Saying you need a brother or a close relative in order to do the transfer? Based on what experiments? The only known operation before Sasuke was when Madara took his brother Izuna’s eyes.