In the next update, Riot Games will focus on nerfing very popular champions: Tristana and Udyr will lower their base attack by two points, Kai'Sa will receive a longer ult cooldown, and Hekarim will receive less speed from his E skill.
Turbo Chemtank will also be reworked. This mythical item will now grant 15% less speed, but slow more upon contact with an enemy.
Alistair and Rell will become weaker in the late game, but these champions will likely continue to be popular.
Yasuo and Varus will gain substantial attack damage per level, while Mordekaiser and Lissandra will do more damage with their Q skills.
Kindred will receive a large buff, at 11.7 she will decrease the cooldown of E skill, and also increase the damage from the basic attack of the Q skill by 10%.
Braum is again aiming for the S-tier of supports: the cooldown of the Invincible will significantly decrease at all levels, and the drop from the ultimate will last longer.
For a complete list of upcoming changes, you can check out the developers' twitter.