Elise is a problem because she does so many things. Either she does everything decently, which makes her absolutely amazing, or she does nothing well, and she ends up useless. You can make it so some parts of her kit are strong and others aren't, but then you get the old Sion situation where you never even use half your kit because it's just there to suck your mana.
The old Sion situation was very different. Half his kit was useless because of the AD/AP split. Elise's kit is entirely AP so there'll never be a point where any of her kit isn't scaling. Elise's kit is split between utility (her Es) and damage (Q + W) but if you make her utility better you're not for some reason gonna make all Elise players stop using her damage attacks. That just makes no sense.
Elise just needs to scale better into the late game. Increase the base damages on her Qs (Neurotoxin maxing at 180 is quite terrible) and maybe the AP scaling on her Volatile Spidering and she's fine.
I see what you're saying, that's a pretty fair point.
In that case, let me ask you, don't you think buffing a jack of all trades like Elise (or Lee) would lower the competitive jungle pool? That's basically the exact thing that happened when she was strong, not to mention she was so good at early diving that is strangled the top lane pool as well.