The sharingan…

The creator of great powers, greater responsibilities, and even greater memes.

Because let's be honest, who can beat the sharingan? Oh? naruto can beat them, I guess you right, what did he need to get past them? Of right, god literally making him stronger.

But first, here is a pretty important skill that will be mentioned later: Observation skill allows to both passively see small details, and to actively see: someone's name (lv.1), stats (lv5) back-story (lv.10), most used emotions (lv.25) and most used abilities (lv.50). They cannot see the stats of someone 50 levels above the user.

Here is how a sharingan skill might look (I will put notes to point out how it would look if leveled up the next levels)

1 tomoe(evolves into 2 tomoes at level 5, 3 tomoe at level 10) Sharingan lv.1 (100 max) (0% exp, 100 for next level):

Eyes passed through bloodlines and told to come from the sage of six paths himself, they give great powers. They say that their purpose is to protect, and their end will be to avenge

Observe skills effectiveness increased by 100% (+100% per next level, max 5000%)

(Unlocked with 3 tomoe): 5% chance to copy enemy ninjutsu as long as compatible with users chakra (same elemental release, kekkai genkai the user has access to) (+ 5% per level, max 95%)

+5 % eyesight cast genjutsu efficiency (+5% next level, 500% max)

(Unlocked 2 tomoe sharingan) All moments seen while this skill is active will be permanently remembered

500 CHA/MIN (490 CHA next level, 50 CHA at max level)

For jutsu copying… well, I would suggest to simply have it give EXP to the elemental manipulation that the ability is from, or in case non elemental chakra manipulation skill (decrease CHA used) with the exception of skills like bunshin, kage bunshin or shunshin, which have unique effects, which should be copied.

Now, let's get to the real OP part, mangekyou sharingan.(NOTE: It has been described to be extremely painful, gamers body makes pain dissipate, its up to you whetever or not the user would be in pain)

Abilities:

Two random abilities, seem to always take inspiration from Asian deities (in case self insert from Europe Greco-roman, Norse or Christian deities could be used as inspiration), the abilities don't seem to depend on the user, and more on the deity its inspired from, but even than loosely (Kamui pulled Amaterasu out of a cave and gave her way back to power, which was translated to pocket dimensions and dimension travel ).

Abilities tend to have one directly offensive and one passive (for itachi, amaterasu is directly aggressive, while tsukuyomi isn't a directly damaging ability. for sasuke, amaterasu directly damages while kagutsuchi shapes the flames as he wishes)

Offensive abilities tend to bypass durability

Susanoo:

*the one who helps at all costs*

(NOTE: every cell in one's body will hurt once activated, again, gamers body might counter)

It creates chakra based armor around the body, first its ribcage, than the arms, head and eventually flesh, skin, armor (seemingly made out of animal pelt) and weapons.

It has a perfected stage, where the armor, instead of a pelt like material, will become samurai attire with a tengu mask. If the user is a Self/insert, the armor might be made into armor of whatever the inspiration comes form; its abilities are based on the deities the user is most closely related to. (the armor CHA uses is only waste due to lack of control, so the gamer ability counters that)

Its weapons can change depending on the user, they tend to either stick to the deity the user is the closest with (sasuke being the reincarnation of Indra, has the arrow of Indra), a legendary weapon based off of deities related to the user or the users abilities.

Mangekyo is also known for making you blind (unless your obito), well that's not a thing anymore thanks to the gamers ability to sleep in a bad and heal all health problems.

Eternal mangekyou is the same as before, with lessened pain and near absent CHA use.