I've had this idea for an action-platformer game for quite some time.
I've been calling it LiGHT, it's a game about traversing the underworld.
The player is tasked with banishing a demon bound within a magic sword.
Think Castlevania mixed with Dark Souls. 2D game.

the players wields a cursed sword. his moveset is fairly simple:
-walk/run/crouch/jump
-slash
-stab
-throw weapon (slash+stab)

i tested it out on paper and it seemed pretty solid.

but I later found that in practice, the combat felt too similar to the game Nidhogg.

Which wasn't a problem, but I felt that it didn't have as much depth as i wanted it to have.
So I came up with the idea of having various playerstates give you access to other moves.

think of it similar to a stance system, based on the players current state.

Tell me what you think of these mechanics:

(While Crouching Playerstate)
Condition: player must be in crouching position
-allows a sort of quick sidestep by simultaneously pressing left or right and jump

(a la wavedashing)
-auto-block towards basic frontal attacks
-unable to attack

(On Landing Playerstate)
Condition: player must be moments from landing
-allows an aerial recovery on landing by pressing jump, which can be followed up immediately by any input,

(Weaponless Playerstate)
Condition: the moments after the sword is thrown, and before it has returned to the player

-allows for weak punches and kicks using slash/stab inputs
-can move slightly faster
-takes more damage
-unable to block

(While Running Playerstate)
Condition: must be in running animation
-can initiate a slide bypressing crouch

(Dire Playerstate)
Condition: player must be under 20% health
-slow health regen up to 20%
-Throw replaced by ranged blade attack
-unable to walk, can only run
-run speed increased

These are only the ones i could think of. much more could be added as deemed necessary.

What do you think? would the implementation of these player-states benefit the game, or would it just weigh down the gameplay?

I would argue that Nidhogg's combat perhaps lacks complexity, not depth, but that's besdie the point. Here are my thoughts on your proposed control scheme:

- You said 2D action-platformer, so I can't help but think of Castlevania. I'm not sure exactly what kind of gameplay you're looking for, but it might not be a bad idea to look at control schemes from newer 2D Castlevanias (Symphony of the Night onward, especially the DS ones. Order of Ecclesia had two attack buttons, like the stab/thrust thing you're toying with), as they're solid control schemes that will be familiar to a lot of people. They go more or less like this: Move/run -> left/right on dpad; crouch -> down on dpad; attack button(s) (one or two); jump button; directional roll with left/right triggers; divekick -> down + jump in mid-air; slide -> crouch + jump; special attack -> up + attack. Attacking is possible from standing, running, crouching, jumping.

- The Player is never denied the ability to attack, except in hitstun and recovery animations.

- Can slide without any momentum. Useful for staying under attacks, or moving under a dangerous area.

- State change of running -> crouching is possible. It is not in your proposed layout, which I imagine could be really frustrating, as crouching seems to be how you block.

- On that note, frontal blocking while crouching seems really strong, as you already get the reduced hurtbox while crouching, and the ability to dodge.

- If "throw weapon"'s input became up+attack, you could give the throw different properties when done with stab or thrust. Like a slow and fast throw on the ground, and 30/60 degree angle when done in the air.

- Having the roll/sidestep on their own buttons means that it can be done out any state. This allows for air-dodges and the potential for cancelling moves with lengthy recoveries.

- The "dire" state seems like not a great idea. Changing/removing abilities of the Player at a time where they have to be extra-careful with their actions seems unfair, and could lead to a lot of frustration. If they GAINED those abilities, like the health regen and the ranged attack, then it's another story.

- The alternate moveset while weaponless seems pretty cool. Would they have to go pick up their sword again, or would it boomerang/respawn in their hand?

- The landing recovery is an interesting idea, but it could lead to some frustration. What if the Player just wanted to jump again after landing? Perhaps this could be only after being knocked into the air by an attack, like ukemis in fighting games (quickly recover from a fall).