Out of all the reworks that I've been waiting for there are two that don't ever seem to be any closer to fruition. These two are Karthus and Nocturn, each for their own reasons.
I'll break down why for each but I'm going to start off with a disclaimer: This is not a balanced discussion. I do not care how broken or balanced these characters are. I do not care about their win-rates or their pro play frequency.
To start, Karthus. I don't think there are many players in the league who would dispute the fact that Karthus's kit is toxic. His Q's do a rather unreasonable amount of damage mid to late game, his wall is one of the largest AoE slows in the game and his W gives quite a bit of mana sustain. These by themselves aren't too bad. It's his passive that makes them obnoxious. Karthus is pretty much rewarded for int-feeding. You dive headfirst into the enemy team, get bursted, and use your wall, W and Q's to deal out a ridiculous amount of damage even from behind. People complained about int'ing sion.
Int'ing Karthus is something I would argue is far worse. A 0/10 Scion might be taking turrets, but a 0/10 Karthus who just crawled his way to a deathcap is going to be chunking your entire team after he's dead. Then you throw in his ultimate. A global, five-man, undodgable, unblockable execute. Which, by late game is going to do 1000 damage on full HP targets. Between both of these, Karthus is nearly impossible to keep shut down.
Karthus jungle is currently a thing because he can perma-farm with his Q and W passive and simply kill steal with his ult once it's up. Suddenly at 20 minutes, you have an 8 kill 150 farm karthus screaming out of the jungle with a death cap because he was able to snipe kills without even pausing in his power farm.
His kit is uninteractive, with counterplay that doesn't feel rewarding. Build a Zhonyas for his ult (Not a mage? sucks to be you), burst him down and then somehow run away from the team fight immediately? It's a strange feast -and- famine type of kit that is just out of date for modern league.
Nocturn has a similar problem. The toxicness of his kit comes in the lack of interactiveness and counterplay, especially with his ult. His ult is semi-global, undodgable, unblockable, unstoppable. There is a small window of opportunity to flash or dash after his ult lands and before his fear does and even then the speed boost he gets will usually allow him to chase you down. The fear is very long, his sustain his high, he has a spell shield. I'll reiterate that I'm not claiming that he's broken. But far too often whether or not you survive Nocturn's ult diving onto you isn't do to any sort of outplay, but because your numbers are bigger than his numbers. Killing someone as Nocturn doesn't feel like "Ah, yes, outplay!", it feels more like "Ah, yes, Stat check!".
Karthus and Nocturn are both horrifically outdated visually. They are uninspired and ugly. Their character backstories are bland and lackluster, with what little stories there are. Their kits also don't mesh well with what lore there is. There is little skill expression and little counterplay. Going on a rampage on either of these champions feels more shameful than it does exciting. A five man karthus pentakill ult makes me feel like I need to go take a shower and apologize to my mother.
A full update and rework of both of these champions would be a solid idea.