My biggest worry with champions being too unique is that riot is going to start making more and more champs that need to be built around. What does built around mean? It means I can't just pick and play my champ. I've got to either hope my teammates will pick according to what my champ does (not happening in soloqueue) or I'm going to be facing a lot of situations where teammates aren't covering for my flaws or building to my strengths.
Take the Sej rework. I'm not going to talk about everything here, but her E has a lot of power now, but it requires you to have a melee heavy team. So at least you are going to want a tank support and ideally a melee assassin in mid lane. Now if you've got a split pushing top who isn't going to teamfight and you've got a ranged support, Sejuani isn't viable. She can't stack her E well enough alone in a teamfight because it's pretty obvious it is meant to have melee teammates around. Then if your top laner loses lane, he's just going to be blown up in a fight and can't really contribute.
So riot wants uniqueness and skill expression. Here's why this is a problem: riot wants us to main niche picks. Why should I "main" a champ who is probably going to be a good pick in less than half of the games I'm in solely because teammates. This is the big problem in reworking champs for Pros only, pros can coordinate well, soloqueuers not so much. I mean it happens, but not often enough. First picking is a risk. Last picking would be conditional on if my teammates need a tank jungler and if there are enough melee people I can count on in a fight.
So on paper, it's a bad idea to main sejuani. For the sake of the argument, let's assume her to be a representative of new and reworked champs going forward. That is, riot wants niche champs who have to be mained. Now generally speaking, skill expression comes after a bunch of games on a champ. So if I'm not going to main a champ, when am I going to get and develop the proper set of skills for that champ?
This is a contradiction.
Riot wants two things that are relatively exclusive for one another.
I feel like going forward, this is going to be a problem.