Eh I'm not sure if I entirely agree on how Volcanion is overlapping Heatran in terms of role compression. Tbh the only main similarity between the two is that both are pretty damn good offensive Fairy checks as well as great partners to the due to their ability to heavily punish Steels. I would say the only reason Heatran might drop off a bit in usage is going to be caused by how you can't really use Heatran on a team with Volcanion without being insanely weak to Terrakion, Tyranitar, Keldeo, Exca, Chomper, and Lando-T, which really sucks. Heatran offers a MUCH more consistent Clef check because it can shut it down with Taunt and trap it with Magma Storm. It's also a better check to more offensive variants of Talonflame, and can switch into Latios a few times if necessary. It's also a hell of a lot more versatile, as it can run all out offensive set, defensive rocks, stallbreaker, Scarf, and even Specs, so it can be tailored better to fit your team. Volc is very prone to hazards, somewhat lesser overall bulk iirc, slower by a few points which can actually be pretty significant (is outsped by Jolly Sharp and Loom if running Modest), and it doesn't pack as many resistances.
Volcanion arguably offers more role compression than Heatran, but it sure as hell doesn't outclass it nor does its job better. Volc will see more usage overall because its more splashable at least until its hype wears off, but Heatran is still a very good Pokemon, it's just that with all the Volcs running around, it has a harder time finding itself onto teams because people don't want to stack weaknesses and would rather use Volcanion + some other rocker or Clef check.
Also I think A+ rank is a very good starting place for Volcanion. Like LittleLucario mentioned it's probably borderline S rank at least right now, but I feel like once the meta adapts to it A+ will be much for fitting for it, maybe A depending on if the meta starts to shift out of its favor, but I can't really see that happening anytime soon. It's so stupidly easy to slap onto a team and the only thing that is somewhat annoying when building with it is that hazard control is pretty much necessary, otherwise Volc has issues coming in and spamming Steam Eruption. Thankfully though it pairs very well with Latios and Excadrill, two of OU's best hazard removers, so it fits very comfortably on sand offense or bulky offensive teams that utilize T-tar.
One of the biggest issues I have with people in this forum is that a LOT of you guys seem to only view Pokemon as if they were in a vacuum instead of looking at them when supported by 5 other teammates. Obviously Volc is slow and weak to Stealth Rock, and it can't solo stall or bulky offense on its own and it is pretty easy to revenge kill especially vs offense. Its main STAB also has low PP so it can be stalled out with something like Suicune or teams that pack cleric support pretty easily. It also doesn't have the greatest overall coverage in the world so it depends a lot on Steam Eruption and the somewhat matchup based Hidden Power. It arguably requires more support than things found in A+ rank like Mega Lopunny or Garchomp. In a vacuum it's far from an A+ Pokemon, but with proper team support, it's easily one of the biggest threats in the metagame and very easy to fit onto a team. The support it needs is usually something that most teams already utilize, such as a hazard remover, stall breaker, check to offense, and something like Thundy or Serp to punish bulky waters. The support it needs is very easy to come by and unlike some of the other big threats in the metagame such as Mega Pinsir, you don't really have to build a team around it in order for it to be successful, in fact you'll most likely find yourself using Volcanion on a team to support something else you're building such as Mega Sableye or Mega Gardevoir.
The Volc meta is still very young, but as
bludz said in the metagame thread, I think Volc could very well be what the ORAS meta needed. While it somewhat forces you to run more aggresive answers to it, it works so well with so much of the metagame that it promotes team diversity and allows you to experiment with a lot more cores. Practically every top tier threat aside from a select few (Azu and Mega Venu took a pretty big hit from it) pairs so well with it offensively or defensively, so much in fact that even pairing it with other waters such as Keldeo, Mega Gyarados, or Manaphy can be very beneficial; Keldeo cleans up very easily with Latis weakened and both Manaphy and Mega Gyara set up on practically every bulky Water-type in the tier (Sub DD in particular). It promotes the use of previously rarer but viable Pokemon such as Jellicent, Slowking, Gastrodon, and Seismitoad.
tl;dr Volcanion is the messiah
