Before we go any further, it's worth noting that teaming is much, much looser than in strictly progression-based games to start with. I've had several teams run without anyone specifically in the role of healer or tank and done just fine, and I've had several task forces declare that we might as well push ahead while the tank is AFK (including one memorable incident when we completed the mission while the tank and healer were both fighting something else, with both groups asking what the hell the other was doing.) But there are still loose targets for where you'd like a group to end up, and still archetypical roles for people to fill.
When CoH first launched, the classes -- well, archetypes -- were specifically designed to feed into the five roles that each party required. Tankers were meant to tank things; scrappers hit them in melee; blasters hit them from a distance; defenders provide support; and controllers hold things. That didn't mean there wasn't quite a bit of overlap or flexibility -- see also scrankers, blappers, offenders, deftrollers, and other portmanteaus defining characters who break the mold. But it's not hard to see that the lines are drawn to encourage those five roles in various mixes.
City of Villains had a slightly different philosophy. Brutes and corruptors were clearly meant to be hybrids -- brutes are equally at home tanking or hitting, while corruptors can be healers or damage. Masterminds, naturally, break the mold pretty solidly, but they function like another hybrid of ranged damage and healing. (Or a hybrid of ranged damage and further ranged damage.) Dominators and stalkers are a bit more straightforward, but they still don't fit neatly into the five roles.
Of course, you still want to aim for all archetypes in the party when you can... but it's not the same makeup. There's no nickname for corruptors who work like defenders, because they're designed for it. Perhaps most tellingly, for most archetypes, what you'll be doing in the group is largely undetermined until the group is actually formed. Your brute may or may not wind up tanking -- with enough mastermind force in the party, there's going to be no need for a dedicated tank in the first place.
Parties are very different on both sides as a result. I've had blueside teams that would wait for a defender or controller, versus redside teams with no real "support" in the party. Oh, sure, we had a couple healing-capable masterminds, but it was mostly a gathering of damage that had some incidental healing backup. And there were very few problems, because several classes are explicitly designed to be, well, self-sufficient.
Yet now any archetype is playable on either faction. Granted, you have to jump through a few hoops to do so, but let's face it: they're not all that difficult. We already know that Praetorian characters can start as either faction at 20, and that's going to make it easy for players who want a villainous tanker or heroic corruptor. And this is going to have some pretty big impacts on how teams are formed.
Strictly speaking, for most teams, corruptors are probably going to work better than blasters -- all the ranged damage with added support. Controllers are better than dominators for a similar reason. Throw in that much incidental support, and defenders are almost wholly superfluous. Brutes can do pretty much everything scrappers can do, and they have the added bonus of getting more powerful the faster the group moves. Seeing as how most groups rush through everything at breakneck speeds, you can imagine which of the two archetypes looks more appealing.
Am I suggesting one thing or the other needs to be nerfed? Not in the slightest. By the same argument, you could eschew corruptors and controllers in favor of defenders and dominators -- incidental ranged damage coupled with control and dedicated support. The point is that the archetypes are going to start mixing together very differently, and not necessarily in ways that benefit veteran players. You might have always been a tanking brute, but a tanker can do what you do with easier aggro holds and faster access to defensive powers.
If anything is going to happen on a macro level, I think it's going to be that teams are easier to form and far more flexible. Villain teams are a bit less common at the moment than hero teams, but that's mostly because villainous archetypes are inherently more capable of performing different roles in different groups. With the spreading of archetypes into both cities, roles are going to become more flexible all around. Defenders and tankers won't automatically be needed for a group, assuming you have the right makeup.
In summary: focus less on role purity as you create your new characters. Have alternate builds for as many different tasks as you think you can perform in a party. Get ready for the game to play very differently... even without the epic archetypes and Incarnate abilities.
I must forewarn everyone that next week we're going to be running a community spotlight instead of anything more strictly focused upon Going Rogue. (The reason has to do with our publishing and proofing schedule, and also the fact that I will not be able to write a thousand words on the expansion after one night of play.) Any noteworthy threads you'd like highlighted, as well as normal feedback, may be left in comments or sent to firstname.lastname@example.org. The week after that, though, it's going to be all about the new expansion. Which is in less than a week. Weird.