Ok so I've read this, and several things
-Heavy as a roamer wouldn't work. Why? Because the heavy HAS to play defensively without med heals, the heavy isn't a pick class and it's very difficult for the heavy to be a pick class. While the solly can.
-Demo as a roamer wouldn't work. Why? Because they can't do close range damage consistently, and have a large difficulty defending themselves.
Thus the Demo and Heavy are really well locked into being pocket combo classes 99% of the time.
Now the Highlander Meta can easily change it with most of the other classes.
-Pyro: a lot of people Haven't quite realized that the pyro isn't just for spy checking, stopping solly bombs, and occasional roaming. Though I think the pyro should do all those things, but I think the primary importance for pyro is to give your team ground combine the high dps and airblast ability and pyro is possibly the most effective push class in medium-short ranges. Add to it damage support from a demo and heavy and the cleanup of a good scout you have a recipe of forcing the enemy out of an area and possibly wiping them in the process. Though the Heavy is going to have to be more aware of the enemy solly and spy in this process. As far as I know -ts- in S8 was and has since been the only team that did this consistently.
-Soldier: Their main role is to watch the flanks, and occasionally get picks. Though some soldiers have been taking a newer approach to this. They've been using the Soldier as a distraction class. Not a class to get picks or really watch flanks, but as a tool of completely annoying and distracting the enemy team, dividing their attention and possibly pulling off 1-3 people from the frontline. Specifically vhalin was really good at doing this last season.
-Spy: As sneakypolarbear pointed out Evan adapted the meta of spy. Evan became a distraction class, rather than a pick class. Though he did get a lot of picks from his revolver, most of what he did was distract the enemy and didn't die in the process. In which his team made plays off of. Rather than the normal spy gets 1-2 key picks, team pushes into area.
-Scout: Pretty much what I said with the pyro, a pyro + scout are really good at taking forward ground and forcing the enemy out of the area. The scout can also utilize unlocks like Bonk to be a distraction or intel gatherer.
As far the other classes Engi MIGHT be able to change the meta with the rescue ranger, but I am unsure how. Sniper idk how they can do much other than get picks and throw piss. Medic meta changes are highly unlikely and would probably be really ineffective anyway.