IDK if this was ever pointed out but i have some theory about MS and why it's different for each uchiha.
Let's start with itachi
we know that he is very skilled in illusions, other than Tsukuyomi, in some pages he is reffered as best genjutsu user known.
he can use multiple genjutsu as easily as moving his finger (in fight with naruto during save kazekage mission)
During naruto - sasuke fight in valley of end we see in flashbacks history of sasuke's childhood and his training with grand fireball. somewhere in this flashback after sasuke's failure his father says something like "no wonder... you're not itachi after all" meaning that itachi at his age was already able to use katon jutsus.
now for sasuke
after countless failures he is finally able to do grand fireball at very young age(remember kakashi's surprised words during survival training "he shouldn't be able/have enough chakra to do this")
next is his shape manipulation skills - i.e. jutsus created from chidori: needles, flow of current, chidori blade(even orochimaru was surprised seeing this)
Lets assume these are their natural talents/gifts.
And here is actual theory
Amaterasu is known as ultimate level of fire jutsu
Tsukuyomi = ultimate genjustu
Some people on forum said that sasuke's second MS skill is manipulation of black flames, like when he put down the flames in B fight and guarding using enton
Does it ring any bell?
My theory is that MS is simply upgrade of one's natural talents to the very limits
comments are welcome and if you find it a total BS let's blame it on "weed talking":D
i have some thoughts about susanoo and latest chapter's Madara's words of where obito is "between this and other world"