First off, no, I don't think we need to rework Kassadin, he is still fun to play and is quite viable in the right hands. However, he needs his visuals, voicelines and even his lore updated.
Let's start with one of the most obvious reasons, Kassadin's character model is quite outdated even though it has been improved before, but that was many years ago, we are now in 2019. Thankfully, Kassadin isn't as "blocky" compared to other current champions such as Malphite and Mundo, but there is still some things that should be adjusted to him.
I wanted his design to still be like he is using technology to resist the power of the void to an extent, and this artwork is a great representation of that (then again it is not my work):
Also, it fixes his "nether blade," Which in game... looks like it is phasing right through his hand.
Another thing I would like to address with this concept art is that kassadin is wearing a chestplate... he isn't shirtless. Why is this important? Well, remember kai'sa and that she didn't have any trace of corruption or scars from the void on her? Well, that's because she was wearing armor, and if kassadin is going to take on the void, it would make sense that he would wear more gear.
Kassadin's voice entirely fits his character... well more like his character model, he is wearing a helmet after all. Not only that, it's epic as hecc.
Voiceline-wise however, Kassadin needs to have them updated. As far as we know, Kai'sa is already out of the void, they need to pull a lucian and update kassadin's voicelines already to interact with Kai'sa. Second, maybe he can have interactions with other void creatures (like specific champions) so we can learn more about them.
We have no clue what Kassadin is doing right now since Kai'sa was already shown to be going in and out of the void. We need a continuation to his lore, because he has so much potential to his character development. That is pretty much the only reason why we need new lore to him because its a bit outdated.
Anyway... that's all of my ideas for kassadin, comment your thoughts.