I one trick morgana and she has several problems:
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Q moves too slowly. It should be faster and thinner with a shorter duration, similar to lux. Right now it is only effective at short range or with a high cc team as a follow-up. It is 100% dodgeable in higher elos at long range.
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Her W does damage over too long of a time period. It used to be great for wave clear, but right now 5 seconds to clear a wave is insanely slow compared to other mages and champions like talon/ekko. It needs to do its damage over a shorter time frame and perhaps do a little bit less damage.
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Her ult requires you to either land a Q or play versus an immobile opponent. Good luck ever ulting people like Ezrael unless the Ezrael massively misplays.
In fact, her entire kit sucks for aggression. She is all about capitalizing on opponent's misplays. This makes her terrible against experienced opponents and brutal versus newer players. The problem is the current meta is all about shutting down your lane, hyper pushing, roaming, and dueling. She's good at roaming, but she can't pressure people out of a lane like a Leblanc can. This means she's not really viable as a midlaner.
I'd say she could be fixed by making her less Q reliant, but truthfully the Q is what defines a Morgana player's skill. I really wouldn't want to turn her into something brain-dead like Garen.