Quirkology

Quirks or even elements of quirks can be classified according to strength as follows:

Detrimental- directly harms or inhibits the user. Quirks commonly have at least one detrimental aspect acting as a limiter.

Vestigial- pretty much useless, or only affects appearance in a non-functional way. Most Quirks have one.

Utility- Quirk has a range of applications.

Weapon- Quirk has an inherent danger to it, or raises the inherent danger level of the user.

Epic- Innately dangerous or uncounterable. A reincarnation/high healing factor on the level of Wolverine (comics) could be considered Epic without being directly weaponizable.

Mythic- unfathomable. Capable of miracles, like resurrection (likely with a time limit, but still powerful), one man nuke, etc. Yaoyorozu might have a move that pushes her into this territory. The Final Solution.

Another thing to consider with Quirks is the Growth Rate/Type:

Mundane growth rates can vary, but are classified by the fact that they can only grow in the same way a Quirkless person could grow in threat level: exercise for strength and training for skills.

Other than Mundane growth, Quirks could grow in capacity in age or by a built in mechanic, and are classified from Slow to Godly. Demonic is also applicable, but tends to be used when there is something... wrong... with the mechanic.

Growth Rate and Type are mostly things to consider for hero candidates, as established heroes are likely to have aged past the need for growth. Another thing to think about when considering growth is equipment compatibility. Some Quirks will increase in growth rate if a suitable tool is paired with them. Consider jet boots on Uraraka, or giving her drones that need less power and noise to function due to neutralized gravity. She could then expand into being a type of sensory hero.

While not directly related to the Quirk Strength, a hero can be classified from F rank to S rank based on a combination of their Quirk Strength and how well they are used. I'd like to call this an Operator Level. As a prime example, consider Mirio Togata of the Big Three recently introduced in the anime. By my reckoning, his phasing Quirk is just a Utility, but his application and precision in using it puts him into upper A if not S rank operator. For another example, there is Uraraka, who also has a Utility Quirk and showed her skill in using it against Bakugou. On the other end of the scale could be All Might, who due to injury, has stepped back in Operator Level due to the time limit incurred.

I think that the average, untrained hero candidate's Operator Level is around C at best. Those who have trained hard on their own could reach B tier, and the most powerful students will be A. S rank would be veterans of the industry, or given to special talents if one factors in age. As for those who have really simple Quirks, like Deku's mom (no offense) due to innate power levels, are rarely ranked. Still, even the Quirkless could be S ranked if trained well enough. Ever read the Vigilante spinoff? Or heard of Batman? They would be S rank operators without Quirks at all.

As far as other ways to classify Quirks, look no further than Brandon Sanderson's Reckoners trilogy, which is also a superpowered society. Want to read a story of unpowered people fighting against a Superpower Apocalypse? Steelheart starts it, and go read it. I mean, if you can read that, or you can read my stuff, by all means, read Sanderson's stuff.

Seriously, go do it. Mistborn is fantastic, too.

Anyways, linked to that is the term "prime invincibility," which is basically a trait that makes someone effectively immortal. Like reincarnation, extreme healing factors, shields, and invulnerability. I'd like to borrow this term.

Also something I am borrowing from the Reckoners is that everything has a weakness. Something that depowers someone, or otherwise can nullify them. All Might has a time limit, Deku has a power cap or he destroys himself, Iida overheats, Uraraka gets sick, Kaminari short circuits, stuff like that. Kai has a couple of weaknesses that are covered in the prologue, but I'll explain that more viscerally when it comes time for it.

More on Quirks:

It is a confirmed though unacknowledged phenomenon in canon that there is a lot of instinctive knowledge that comes with having a Quirk. Take Toga (drinks blood for shapeshift creepy girl) or Stain for example. Without instincts driving them to consume blood, they would have gone through life thinking they were Quirkless. Clearly, procedures that come with activation of a Quirk are ingrained into the user.

Obviously, that is a little different for Deku, who has to figure things out. Things aren't as natural for him. And One for All is a unique Quirk to begin with.

It is confirmed that there are Quirks that operate directly on Quirks. Three examples are given: All For One, One For All, and Aizawa's Eraser Quirk. While these would be rare, they are clearly not unheard of. Just having a Quirk that operates directly on other Quirks might put your power level into Epic, but with limitations considered, that might be difficult.

Well, I'm sure I'll cover some of these within the story, but I wanted to get this into a convenient place for people that want it.