"Meta" is a concept that forces people into certain roles when searching for public groups, and players have shoehorned themselves by following the "meta" that is exercised by the top groups - even though for lower level of play it's not necessarily the best thing to do.
When you play with friends, or when you don't play in 4 player groups, there is no meta. Everyone plays the class they like. There are certain synergies (like buffs being added to the group, pulling, giving defense/offense bonus or movement speed) but at what cost? In 2 player it really doesn't matter, and even in 3 player not that much. If you feel you hit a wall and want to push further, you can always switch classes.
Especially after this balance patch no one knows what's "overpowered"; everything seems to be relatively even, so the meta is that there is no meta; currently, the meta is a mental block in people's head to avoid them following their preference. (There's almost never a 2 player meta, btw, not even for the top of the leaderboards; there are better and worse combinations, that's it, but currently we don't know either.)