Gen 3 impact

No Impact Golem is still the Legendary Bird killer.

Weather is the bigger change by far.

I don't wanna go all hairytripper2 or w/e his name is, but there is little meta to speak of that has room for change.
The top spots for some attackers might shift by 1 or two when legendaries are released, but I doubt any 8/10 and higher from gen 1 & 2 has become obsolete due to gen 3.

Weather is far more significant.

Personally, I think the weather system does more to impact the meta (at least the raid meta) more than the Pokemon themselves.

Soloing 3-star raids is going to be a lot easier. The new weather system is an indirect nerf to Omastar since Exeggutor suddenly got easier to obtain at high levels and gets buffs in clear weather to its Grass moves. Scyther is also easier since its main weaknesses have some of the most common weather patterns (sun and partly cloudy). Ninetales is harder to beat now, but it is not as desirable as, say, Machamp and Alakazam. Alakazam may also lose some relevance as a Machamp buster now that we have Gardevoir, but Alakazam's availability buys it a lot more time.

I'm also disappointed in the lack of change. Would at least have expected for some of the harder to acquire new mons to be better at something than what we already had available.

Slaking is indeed a trophy, Salamence is looking like worse than Dragonite according to simulations.

With Gardevoir, Metagross (soon), and the advent of weather changes, Fire-Types will have a more developed place in the meta. The new moveset for Blaziken challenges Machamp for fighting supremacy, adding rivaling damage rate at the cost of a late damage window and a 1-bar move, as well as key resistances to Fairy and Steel.

I agree with you for the most part. Slaking doesn't seem to be as useful as people claim it is, only being useful for adding to a Blissey gym. For the most part, the meta-changers will have to wait for the full release of Gen 3.

Salamance won't be too different than Dragonite, though the quick one-bar charged move it has might put it ahead of the 'ol friendly dragon.

Metagross and Aggron will both thrust Steel Types into the meta, where currently Scizor was struggling to do so. I can't say for sure that Metagross will outpace Scizor's Steel DPS despite its much higher stats, since its moves across the board are pretty mediocre for their types, but it absolutely has the defensive stats and typing to back it up. Aggron will be a MUCH more useful Steelix, especially with its attack stat, but only for the uses of attacking with resistance or double resistance. It's typing for double resistance is the same reason that it won't defend quite as well, as it boasts two double weaknesses that are common in the hands of experienced players (though Aggron as a defender would be a nightmare to a newer player).

I think Gardevoir's contribution is more so as a gym defender than an attacker. Confusion and DG are both renowned for being deadly defending moves, and it's typing will absolutely force a switch from fighting type sweepers. Its hard counters are limited to Steel types like Scizor, Aggron, or Metagross, and really strong fire types could do the job too with more difficulty.

That's all I can see though. If that's not enough, that's because it will take more than some new Pokemon to change the meta. It will take new mechanics, such as active PvP.

Gardevoire is about to be in every gym. I was lucky enough to catch a level35 15 atk moderate IV Ralts. I evolved it to a 2680 Gardevoire with C/DG. She resists pretty much all the main gym sweeper types and is o my weak to some fairly uncommon ones.

Even before Gen 3 was released, the current attackers list has 10 out of 13 S/A Tier from Gen 1 (8 out of 10 non-legendaries). With Espeon very similar to Alakazam and Raikou only slightly better than Zapdos, Blissey and Tyranitar were the only really significant Gen 2 contributions to the meta.