Champions Have Roles -- And They Should Stay There.
One excuse Riot often uses when it comes to balancing certain champions is that they are too strong in other roles if buffed for being weak in their primary role. Non meta and unusual champions for role picks has long been a problem in League of Legends. Sometimes item changes make certain champions really strong or safe in other lanes (i.e. Lulu Mid / Top, Brand Support) and those picks become really frustrating to play against. Then we have the opposite side, where a player happens to really enjoy a certain champion and decides he or she wants to play it everywhere on the map. This more often than not is frustrating for the rest of a team, because another champion is much better suited for the role, and it's not always easy to adapt and play around something that is rarely seen. Both of these situations are not healthy in League of Legends, and Riot has done little to nothing to actively address the problem.
Each time Riot has released a new champion, they state their intentions along with what role the champion has been designed to play. As of recently, you can even sort champions by which role they belong to. Riot has even gone as far as removing parts and toning down champion kits to ensure that they cannot Jungle very well (Alistar S3, Camille PBE, Ekko's original passive, etc). To me, this is a big indicator that champions are not meant to be played outside of the role they were designed to be in. Why go to such lengths to pressure players into picking only certain roles, without actually preventing them from doing it?
At the end of the day, I really believe players should only be able to select from champions in their intended role during champion select. Some players might be upset at this notion, but generally it's just not healthy in anyway for champions to be played everywhere. It forces Riot to make additional balance adjustments that otherwise wouldn't be needed, often it's not fun to play against, and sometimes it's not fair to a player's team to be forced to play with. Marksman do not belong in Top lane, Supports do not belong at Mid lane, Karthus is not a Jungler, and Yasuo is not an AD Carry. Every champion has a place where they belong in League of Legends and it's high past time Riot started enforcing it.
tl;dr
Remove the ability to play any champion in any position.