I am all for creating strong niches for existing champions, but...

... i am skeptical about slapping those niches on already strong, popular, all-round picks.

Example: Renekton is a very effective bully, and has been in the past very often too. He generally trashes tanks, and tanks make use of shields. For him to get a shield breaker sounds like overkill. Renekton doesnt need it to do his job.

I think that an extra mechanic that solidifies a niche should be introduced on a champion who actually needs something extra to be picked and played.

For instance, i think that the shield break would be an awesome thing to have on Wukongs Q. Wukong already struggles in the toplane against a wide variety of champions; giving him a shield break would make him a viable counterpick.