The only nerf I agree with strongly is the reduction/removal of invincibility frames on vanish. It makes going for an airdodge read with it after d throw at high percents much riskier, as the opponent can simply throw out a hitbox, and it also makes going off stage more of a commitment for her; edgeguarding and off stage play is cool, but the fact that there's such little risk for doing it with Sheik relative to the other characters is lame.
Nerfing needle damage could work, weakening her command in neutral at range because getting hit by them isn't as big of a deal. Adding end lag to remove a cool combo is, well, not cool.
I don't think giving her the old bair back is a good idea either - she doesn't deserve a reasonably safe kill option. She's the classic example of a fast character that can hit a lot, but each hit is less damaging, and struggles to deliver the finishing blow.
Adding lag to fair ruins her. Sheik pretty much is fast fall fair; it's what makes her great, and without it is she is completely different; indeed, it would very much change the 'feel' you intend on keeping. It would take away combos, a spacing tool, so much. It by nerfing fair, not only do you reduce it's effectiveness, but you also make it less threatening., removing a layer of mindgames in neutral.
I'm still very much an advocate of buffs rather than nerfs. Why not give other characters safer options, or give them much greater reward for landing high-risk moves? Wario, for example, doesn't get much reward for hitting an upsmash, which can't be combo'd into and lands mostly as the result of a read, or well thought out option-coverage. He's not the best at racking up damage, but depends very much on the fart (and/or the threat of it) to take a stock. Perhaps he deserves a better, safer KO option, and Wario isn't even a low tier character - actual low tier characters could see much greater improvements, making them more fun and less punishing to choose to play.
Sheik isn't really close to brawl MK. Given time, and huge character development, people found methods to deal with what seemed to be broken. Regardless, he continued to dominate tournament placements, which made a solid argument for him just being too good. This hasn't happened with Sheik - in fact, she's not really winning so many tournaments at all!
Relying upon nerfs to top tiers hinders meta-development of lower tier characters. The hope that "She'll be patched next" stops one from working hard to find methods to deal with her. There will always be a 'best character', but the job of the balance testers is to make it hard to determine - to make the creation of a tier list difficult. This would involve shifting the characters to a certain standard - why not make that standard high?