Hey! ^_^ Sorry been a bit busy lately.
Anyway here are some ideas I had, take what you want from them and use it as you please or trash the whole thing if you feel like it.
For this concept I'm going to try and keep her as utility mage who specializes in binding
Passive:
Q- Creeping Dark- Nyxa releases one of her shadows as a skillshot, if the skillshot hits the shadow tethers to the target dealing 10/35/70/125/230 (+60% ability power) and revealing them if not stealthed , if the tether remains for 2/3 seconds the enemy takes another 10/35/70/125/230 (+60% ability power) damage and is possessed by Nyxa and attacks allied units/snared/stunned.
With this ability I tried and combine your q and your w while adding a bit of counterplay.
W: Malicious Intent: Nyxa unleashes her shadows in 300(?) radius area for 5 seconds dealing 10/25/40/65/100 (+40% ability power) per second and slowing them for 10%/15%/20%/25%.
Synerigses with your q and ult , use it to punish people for getting tethered or to get your tether.
E- Black Mantle- Nyxa pulls her shadows around her stealthing herself for 3 seconds, if she is out of combat it takes 2 seconds to completely stealthed if she is in combat it take 4 seconds. While stealthed Nyxa gains 10%/15%/20%/25%/30% movespeed. When she's stealthed near a certain distance from enemies she laughs warning enemies of her approach.
Changed the e a bit, removed the shield and turned it into a generic stealth. Added the laugh because Nyxa is incredibly powerful when stealthed and nobody like being blown up by a stealthe champion. Just look at rengar, shaco, and old eve.
R- Shadow Bind- Nyxa prepares to bind the shadow of her enemies, she marks the ground in a 500 radius circle, after a 0.75 delay all champions have their shadows binded to the center of the circle dealing 100/200/300 (+50 ability power). if the enemy leaves the circle they take 250/350/450 (+70 ability power) and are slowed for 80%. This bind lasts for 1.5 seconds.
Kind of wanted something to say that she's the shadow binder. Kind of copied Puck's ult from Dota 2 though:/
I don't know what role you wanted Nyxa to fill so I turned her into some kind of ganking/utility mage. Her goal in the game is to roam with her invisability and try to snowball the other lanes with her e and snaring them with q. When mid game comes around she's going to be looking for a good ult opportunity or try to peel off enemy champions with her cc.
I don't know how to do numbers so they are probably pretty off:/ and i don't know if you want to keep the infinite scaling or not so I didn't make a passive.
Anyway this was my idea, feel free to do whatever you want with it. If you could provide me with more information in the champ I could write some lore if you would like.