Yeah, Karthus seems to be very weak in lane, but even so I've seen a few fed Karthuses that can melt anyone that comes close and then rake in kills by ulting at opportune times. Regardless, the main reason I was looking at him is because he seems to contrast nicely with Malzahar, letting me have two completely different mid laners with different functions that can cover very different team comps as necessary. To compare:
Malzahar:
+ Ludicrously strong in lane and in dueling
+ Has no real hard counters, with his biggest problems coming from long range poke champs with no kill pressure
+ Massive burst damage can 100-0 about half of champs when on even terms
+ Pushes well, farms well, takes objectives well
+ Fairly versatile in terms of builds (personally, I alternate between a more "tanky Malzahar" build centred on Rylais, Zhonya's and Liandry's and a "hyperfed carry sustain Malzahar" centred around Athenes, WoTA and Rabadon's
+Lots of utility thanks to his extremely long silence, his max HP shred, his voidlings and his suppression
-Carrying potential limited by the fact that he can only exert pressure on other lanes/jungle if he's already safely secured his own lane
-His late game is pretty good so far as burst mages go, but he certainly falls behind
-No mobility to speak of, and unless he's fed he can only take on enemies 1v1, meaning he's vulnerable to ganks/jungler camping
-Not quite as strong in teamfights
vs Karthus
-Squishy, more or less short ranged
-Very weak in lane, hard countered by assassins, no burst at all
-No mobility
+Strong late game
+Can exert pressure on other lanes without dominating his own
+Very high damage and effectiveness in teamfights
+Basically infinite carrying potential (in the right hands)
I might be wrong about Karthus, but I think he has sufficient carrying potential for what I need.
Anyhow, here's the situation in more detail; I recently hit level 30, and want to do well in my placement games. However, I have very few champs that I actually play regularly, those being, more or less in order, Malzahar, Twitch, Teemo and Heimerdinger. Teemo is only really useful in specific situations (eg. top lane vs someone like Tryndamere or support against a short ranged adc) and recent changes to ADC items have made my old reliable Twitch build/playstyle seemingly uneffective (specifically, rushing BT and going for very aggressive ambushes using my long stealth), so that leaves Heimer and Malzahar as champs I play regularly and am good with. Heimer has basically no carry potential at all, so I've been working hard on getting good enough with Malzahar that I can carry whatever team I get to victory. Indeed, this seems to work about half the time, with many games where I go 18-3, roam enough to help my teammates win lane, and then win very quickly before our team's incapability of teamfighting becomes an issue. However, I've run into the following issues that make Malzacarry tricky:
-If I'm laning against someone that's very cautious and has some range, I can't leave lane without them hammering the tower. So I'm basically glued to lane as other lanes are slowly lost to attrition, until we reach late game where I'm the only one capable of dealing damage, I'm not fed so I can't carry, and then I get focused and die.
-If the enemy jungler knows what he's doing, he'll recognize that Malzahar is ludicrously strong in lane and can get fed early if he doesn't help mid out. So he'll camp the hell out of mid, turret dive (with mid support) to guarantee kills, and generally besiege Malzahar for the entire early game. When this happens to me, unless I get very lucky with timing and manage a double kill the usual result is that I'm dependent on my jungler to make up for me having no global presence, no kill pressure and inadequate farm, with the usual result being that he does nothing and we lose in late game.
-Some teams are simply too bad to carry, simply because they don't understand when to initiate a teamfight, how to focus in a teamfight, how to play cautiously if behind or how to take other objectives when the team is behind. I've legitimately had situations where half the team (consisting of mid silver players) was focusing the support in a close team fight because "she's low", and where I ult combo someone wreaking havoc upon our team, but they simply run away instead of finishing them off.
These are all things that have happened in games I've played, by the way. Hell, all were exemplified in the last game I played.
So I started the game and discovered I was laning against Yasuo. "Oh boy!" I thought to myself, "I basically hard counter Yasuo, this should be easy!"
So I started laning and discovered that this Yasuo was quite aggressive. He engaged in many trades with me, especially when my E was down or my mana was low. I generally came out ahead, and figured that I'd wipe him out come level 6. Suddenly, Lee Sin jumped out of the bush, beat my face in along with Yasuo, and secured a kill.
"Damn, I was too cocky there," I thought, "I'll play passively for a bit until I can take Yasuo." So I continued trading hits with Yasuo, but avoided going too far ahead so as not to get ganked. At one point, Yasuo got my health very low, but I got his very low and he appeared to be backing. So I went to my own turret to back. Suddenly, Lee Sin jumped out of the jungle behind me, killed me, and escaped as Yasuo returned to lane with a health pot. So I was basically shut down early game.
After another inconsequential 1-1 exchange with Yasuo, things turned around as I began roaming. I rescued my Wukong from Teemo and decisively turned the lane around, to the point where Wukong could beat Teemo on his own, and then I won a good team fight in bot lane. By mid game, my KDR was something like 9/3/5 and our team comp seemed to be effective in all the team fights. However, right before the most important teamfight yet, our Wukong said "brb" and left for about three minutes. Ignoring this, our bot lane (responsible for the score being even close, by the way, having basically fed all game) charged into a very pointless 4v5 that we lost, and it was all downhill from there. Wukong arrived in time to see us lose two inhibitors, but by that point there was little we could do even with a functional team comp.
So yeah, point is I want a mid champ that works with team compositions Malzahar might not work with, that works nicely in teamfights and can carry very, very hard, or at least can affect other lanes early on without necessarily having to dominate mid first.
Oh, and on a completely unrelated note, how's Zilean? Most of what I've seen is that he's close to being a ridiculously high mana champ that can't burst or poke and only has a few tricks, but I've seen a few people claim that he's highly underrated and works very well in mid lane under certain situations.