k, what does she do post laning phase? every tank has some form of cc, she won't, all she can do is burrow in and die because no knock up, she can't peel for her team, she can't do anything.
This is an odd one because the correct path here depends on how you define what Rek'sai is meant to be. Riot released her as a fighter, not a tank.
I'd agree that removing her CC straight up is extreme and would most likely kill Rek'sai, but if you agree that Rek'sai should be more of a fighter it's not the worst direction you could take. If you were to entirely remove her CC and give heavy damage compensations you'd wind up with a Shyvana like Rek'sai who trades stickiness and durability for vision and mobility.
If I were to work on Rek'sai personally what I think I'd look at is removing the knockup on unburrow and replacing it with a ground (Cass's W effect) and adding a stun to her E which scales up with the amount of fury that she has built up when she uses it. You would probably also need to increase Rek'sai's damage doing this but I can't playtest it or anything so it's pretty much impossible to say how much by.
My reasoning for this is to limit Rek'sai's CC spam a bit, it gets insane when she's knocking up a different target every few seconds because he CC ability also has the shortest CD on her kit. I'd also like to see more use for fury on Rek'sai's kit, compared to Tryn, Ren and Shyv she doesn't really seem to care that much about the amount of fury she actually has built up. The ground also makes it more possible to turn and fight Rek'sai which is a positive IMO.