They're going to 10 bans next year, so that'll help diversity too. The top tier champions are top tier because they're incredibly well-rounded. They do some things better than others, but do nearly everything well. Being able to ban an extra 4 champions means that the ban phase will be a lot more important and will bring a lot more diversity. If you can ban the 10 most well-rounded champions, we can see more specialized picks come back. Take Yorick, for example. Reworked Yorick is a specialized split pusher, the best in the game, but that's all he does unless he builds his ghouls up first before a teamfight or summons the Maiden in the teamfight, rather than sending her down a lane to split. However, a comp designed around allowing him to split push and having an incredibly powerful teamfight without him, for example, Amumu jungle and Miss Fortune ADC, can allow him to do his 1/4/1 solo split, putting the Maiden in one lane while he goes down the other. You really can't get that kind of strategy, though, without the bans to counter it. When the enemy is able to run a more well-rounded top laner, who is both able to split push and to kill Yorick, that strategy can't be run.
Another thing that would help diversity is champion balance as a whole. Like I said, the top tier is top tier because they're so well rounded. In season 5, this was Azir and Kalista before Riot gutted them. Now, they might get some small buffs in the preseason or early season 7 so that they're at least playable for people like me who find them fun, but they're pretty much going to stay gutted long-term until Riot can do a full rework on them. Nidalee will be overall weaker in the new jungle because she just won't have the sustain and AoE to clear as fast or as well as she does. Graves will likely be just as strong or stronger because he will still have that incredible AoE clear that he comes out of so healthy. Seriously, though, nerf Graves. If you're going to turn him into kinda a bruiser ADC, that's fine, but I don't want to get killed by 2 of his autoattacks before he even builds crit when I go in to assassinate him.
Sorry about that bit of salt. Anyways, 10 bans and better balancing the champions to get rid of these incredibly well rounded picks who really just don't have viable weaknesses would both help the champion diversity.