Is there a way to fix underused Champions?

Hello fellow Summoners,

Today, I want to talk about Xerath, along with many other champions to see why they aren't used and what changes must be made to make them attractive. First, I want to talk about Xerath. Known for being capable of bursting enemy teams from 1200 units away sand remaining safe. At one point in time though, his Ult was completely useless and he was unusable. After a few buffs and fixes, though, Xerath was gaining popularity... or so it seemed. One problem Xerath has in his gameplay is having no mobility. He freaking locks himselfs down for crying out loud. This does more harm than good. The 2nd problem Xerath faces is that 2 of his 3 damaging spells are skillshots, both being easily dodgable. They do tons of damage, but so what if you can only hit 1 out of 3 Qs or Ults. Finally, Xerath is super reliant on his ultimate. With it, he deals tons of Damage. Without it though, all he can do is poke and probably stun someone.
The 2nd champion I want to discuss is that raging Alligator, Renekton. Has a strong, if not the strongest early game out of all top champion, yet he isn't used so much due to how he falls off late game. One reason for this drop in power is when Renekton is on lane, he is able to farm out his rage bar and deal nasty damage with fury empowered skills, but when it comes to teamfights, it becomes really hard for Renekton to stick to targets and gain enough fury to dish out enough damage. I would say how I would fix Renekton, but I'm no pro with him, so I leave it to you to find some genius change that would boost our Alligator into Fotm.
Farewell Summoners, and remember, think Against the Meta.
P.s.: If you have nothing relevent to say or don't like what is said here, move along. There have been way too many times where some guy comes out of bleeping nowhere basicly just to say, "You suck, you Antimeta meta sheep."