Okay firstly, if you are one of those people who say "if it isn't broken don't fix it" Get out because if you are saying that you obviously haven't been looking at any of the champion updates that Riot has been doing. Because while some of the champions who have been updated need it. Most of them didn't.
Anyway, Anivia is a very basic champ. I don't mean that in a way that she is easy to play. Only that she hasn't got hardly any versatility compared to other champs. She has a good early game with her powerful poke damage and her rebirth. But unless she is incredibly fed or the other team are just really bad she isn't gonna do much. Sure, if anivia gets fed she can easily have a great game. But with the mid laners there days and the amount of counters she has that is rarely the case. And most of the time she just dies by the pure lack of any health or reasonably good escape.
Yeah her W is good but if the other champ has any sort of jump, dash or blink with almost every champ has these days and 90% of mid laners do, it is not gonna be really effective. Her Q is easily her best ability, her ult is worse than her Q just for the fact she needs it to do anything because if you don't lock the other champ down with it they will rip your small health to shreds. Her E has one use and that is just her combo, If you don't have the combo using it is not effective. So it is just an ability to support another ability you can't use it in normal combat because then it is jsut a waist. And her ult. While yes it is sort of effective as it lets you do your damage on your E but other than that is is just a small damage source for a short amount of time. The slow it applies is practically nothing and while it does slow attack speed it is barely noticeable. And ultimate ability is supposed to give you a huge advantage during a fight. But have her ult a damage over time effect when the only sort of damage you can do is burst is doesn't make sense. Yeah it has a short cool down and that maybe makes up for it but if you are fighting any other mid laner or jungler which are the two you are most probably gonna come into contact with it doesn't do much. Her ult is basically a farming tool because she cannot farm well pre 6.
I am not saying she is a broken champ because she isn't, she is reasonably okay and if she is fed she can carry a game. But most of the time she isn't. I believe she needs an update. Not a massive one but at least something that makes her able to have more versatility. But if you disagree say why. But like i said, if you say "If something isn't broken don't fix it" i will ignore you because with the recent champ updates that argument is invalid, useless and just not good.