So, let's look at interesting tier changes that may be coming up
OU vs UU
Roserade is well above the OU cutoff, so unless it takes a huge dive in September, Roserade will be leaving UU. This will have a huge metagame effect. Most interesting is that spiking duties will have to fall to Omastar, Qwilfish, and Glalie.
Alakazam is a bit below the OU cutoff. A good performance in September would probably save it from UU, but it has a very real chance of joining UU. The effects this could have is unknown. Zam has both good offensive and good supporting options, but Sucker Punch and priority is everywhere in UU, which may give it issues.
Rhyperior is quite a bit below the OU cutoff, and will need a fairly strong performance next month to keep itself out of UU. In UU, the large number of Water and Grass attacks flying around may keep it from being too big of a threat. However, Rock Polish Rhyperior would be a deadly addition to a Sandstorm team, and CBperior could probably do well, too, what with its 624 attack and Edgequake STAB. Also, the Trick Room players (ie, me) will love him. 140 base attack and 40 base speed...*drools*
Porygon2 looks like it won't be joining OU unless its usage takes an upswing next month. Of course, Porygon2 was never a major player in UU, so this probably won't matter.
Umbreon has fallen in usage. If this trend continues, it's likely it will rejoin UU. Again, Umbreon was not a huge threat in UU, so this is unlikely to be a major change.
Heracross usage fell again. Though he's unlikely to become UU next tier list, it's very possible in the future.
UU vs NU
Charizard has been losing usage, and has been below the UU cutoff for the past 2 months. It is likely it will join NU, adding yet another dangerous offensive threat (BellyZard, anyone?). I don't think any wall in NU can stand up to BellyZard, and Jumpluff is one of the best ways to set it up. This could get interesting.
Nidoqueen lost a ton of usage this month. She could bounce back, but she may be joining NU come October. Her defensive presence could be very useful in the tier, with some nice resistances, good support movepool, and Tspike absorption.
Typhlosion also lost quite a bit of usage, though it's still above the UU cutoff. But if it continues this trend, it will wind up in NU. When it comes to all-out offense, Typh is an even better Entei, with more powerful Eruptions for early game and Blaze Fire Blasts for late-game. A potentially very interesting addition to the tier.
Similar to Typhlosion, Magmortar took a huge hit. I'm starting to suspect Dugtrio is responsible for this. Magmortar is a pretty nasty mixed sweeper, and will likely be a major threat in NU should it fail to make UU.
Medicham has been hovering around the UU cutoff for the past two months. In July, she was above it, and in August, she was barely below. She may well find herself in UU in October, which would certainly take that load off the NU metagame.
Drifblim will probably to to UU. It's been well above the cutoff for 2 months. This will leave Dusclops, Sableye, and Shedinja as the only defensive ghosts in NU.
Leafeon and Magneton's usage exploded since July. If this holds, they'll both be in UU come October. Magneton means Probopass will have to take up steel-trapping, and with Leafeon and Tangrowth both in UU, Meganium will have to be the go-to for defensive Grasses.
However, they're nothing compared to Kangaskhan, who saw probably the biggest increase in usage ever, and will be certainly leaving NU by the next tier list.
Lastly, Porygon2's usage has been falling, meaning it's likely to enter NU next tiering, though a strong performance could keep it in NU. NU Trick Room would probably love the extra TR starter, and Download would help keep its threat level up. Trace can Intimidate Tauros and grab Water Absorb or Flash Fire to wall their respective users, and can trap Probopass, though you'll need HP Ground/Fighting to beat it. Additionally, Tracing Wonder Guard could be game-ending, though you'll want P2 to carry Toxic or a SE attack, for obvious reasons.
After looking at these NU changes, the metagame looks more fun than ever. We really need NU ladder on the main server. Perhaps once stage 3 is over, a NU ladder can replace the suspect ladder.
EDIT: I figure I may as well put in my predictions for the tier list changes come October.
Roserade -> OU
Rhyperior -> UU
Alakazam -> UU
---
Kangaskhan -> UU
Leafeon -> UU
Charizard -> NU
Magneton -> UU
Nidoqueen -> NU
Porygon2 -> NU
Medicham -> UU
Magmortar -> NU
In order of decreasing likelihood.