Patch 7.5 contains substantial changes for the professional scene, as did patch 7.4 before it. The Big Three are the largest targets, but there's also plenty of nerfs directed at popular champions like Varus and Jhin. Unlike the previous patches however, 7.5 might shake up the professional meta and bring new champions into the scene.
The Big Two
Camille somehow slipped past the nerf bat in 7.5, but her fellow companions in the Big Three didn't. LeBlanc received a fairly substantial nerf to her Distortion's damage (it's been cut in half, with reduced scalings to boot) and Rengar took a hit to his W's sustain against jungle monsters.
The Rengar nerfs aren't likely to affect his play in the LCS much - after all, he's already dropped down a tier or two from his days in the Big Three. However, the LeBlanc nerfs are significant, as she'll no longer be able to use Distortion to chunk entire teams or brute-force damage onto her lane opponents. Being forced to rely on her unreliable Q-E combo might be what tosses LeBlanc from the meta entirely, though it's more likely that she'll just be a less-important pick.
Marking down marksmen
Varus and Jhin have been the top-tier ADCs since the beginning of the season, and Miss Fortune has quickly risen to join them up at the top. Accordingly, all three received nerfs in some form or another - however, it's Varus who suffered the most. His Piercing Arrow's cooldown has been increased and damage decreased, though he can restore at least some of the cooldown of the ability if it detonates Blight stacks on enemy champions. This pushes Varus away from his fire-and-forget playstyle and forces him to either wait longer between Piercing Arrows or get more involved in teamfights. It likely won't be enough to remove Varus from the A-tier of ADCs, but will likely knock him down a peg in draft priority.
Miss Fortune meanwhile has lost damage on her Double-Up's bounce, but gained the ability for the second hit to critically strike. It's a small change that mostly nerfs support Miss Fortune, but is likely to reduce her ADC effectiveness as well. Given that support MF is a fairly niche pick as-is, she may drop from the meta entirely now that she doesn't have the ability to relentlessly bully enemy laners.
Finally, Jhin was nerfed in a very indirect fashion - Deathfire Touch, his preferred keystone mastery, now has less AD scaling. Essentially, Jhin has to either choose between dealing less damage than before with his abilities, or pick an entirely different mastery. It's a pretty significant nerf but one that likely won't affect his playrate in LCS, given that he's mostly chosen for his ability to set up long-range plays with Curtain Call and Deadly Flourish.
Mastery swaps
Warlord's Bloodlust - long the go-to keystone for ADCs - and Bond of Stone have both been significantly changed in 7.5. Warlords in particular is of note, as it grants significantly less lifesteal and must recharge in between lifesteal-augmented attacks. It's a pretty substantial change and may push most ADCs to opt into Fervor of Battle instead. This might have the side effect of pushing out traditional ADCs in favor of those who can exploit Fervor most effectively, like Lucian or Sivir, but it's more likely that traditional ADCs will just swap over.
Bond of Stone meanwhile has been completely replaced with a new keystone mastery, called Stoneborn Pact. The new mastery grants bonus maximum health to its user and allows them to mark enemy champions with a sigil that heals allies. It seems quite effective on health-stacking tanks and supports and might see play in the LCS, though probably only on wardens like Thresh or Braum.
