Although I don't think buffs should be given haphazardly, I completely disagree with dark sonic in this instance. Characters that are good should not receive buffs. No. But, characters that are decent or sub-par should be buffed. There is a lot we can do to balance characters. There's also a lot we can do that would easily make one overpowered. However, I think the easiest and most effective way of balancing this game would be to distribute buffs to the characters until we feel they are all "high" tier material. If a character feels like they're mid tier, then there's something wrong. Mid tier basically means "barely usable" while low tier is "unusable at all." High tier is what we're really looking at when we talk about a character being "viable." Of course, we would nerf the obvious tops such as metaknight enough to bring them down to high tier.
Also, who says that high combo power and kill potential together makes for a broken character? Look at falcon. He can combo people like crazy into a knee, which kills people at like 90.
For the people who are wanting a sheik buff in return for the ftilt nerf... I don't think this is too unreasonable. I think that giving her fair the killing power it had in melee is unreasonable, however. That thing killed you in melee at 90, even with good DI. Unlike falcon, shiek has some actual priority, so this would be too much of a buff, I think. If the move killed around 120% or so with good DI then I think it would be perfectly fine and keep the character balanced. It doesn't kill until at least 150% right now. What might actually be a slightly more appropriate buff would be to raise the uair's and nair's base kb. Both of these moves have sweetspots that last briefly that are getting fixed to begin with, but the hits themselves aren't that strong. This would buff her ability to kill on either of these moves by no more than 10%, meaning opponents will still have to be up at least at 120% or so to get killed, which I think is actually a fair percent to be killing at. Even characters who have "bad" kill moves should be killing by 120% on a regular basis imo. This would also compliment her "combo-oriented" game, as currently you can be punished for landing these two moves at a low percent if you try to follow up.
If you're going to argue that players should be forced to switch to zelda for their kills, then... screw you. If you want to play as sheik, play as sheik. If you want to play as zelda, play as zelda. We should not be making people want to use zelda because they need her in order to get kills as sheik. We should be making people want to play as zelda by making them want to play as zelda. They are their own separate characters, and should be treated as such. This means that neither of these characters should be neglected just because "they have their other half." PT was fixed to be able to operate the pokemon independently. I don't see why we shouldn't do the same, here.