Not playing much PvP right now, but I see quite a bit of diversity on PC EU. To some degree, I think that may be due to people not moving on from their old setups. I see magsorcs with conventional playstyles that include Streak, Meteor and shielding. I see pure stamsorcs. I see hybrids using Crushing Shock on more stamsorc-like setups. I've talked to a nightblade doing fine with old sets, e.g. New Moon, Clever, Balorgh, Markyn. I've seen other (bow) nightblades with poison setups and Kjalnar.

Of the sets you mentioned, I find it hard to get away from Rallying Cry as a solo player, but I fail to see the reasoning around Sea Serpent that people use to justify that set's OPness. How many actually use it? Movement means a lot to me. People say you can claw back movement with Swift, CP, and so on, but IMO the average PvPer is not sitting at 0% movement buff. You're competing with players that have +10% to +30% as standard (in combat, in CP) before we even get to the outliers. I find this noticeable on melee templar, for example. If I can't at least match the movement speed of my target, they can and will frequently outrange me. I think Sea Serpent may be meta for people who are already used to a slow playstyle, perhaps a DK who lets people come to them and leaps to execute, or even a nightblade that relies more on Shadow Image than speed, but it's not for everyone.

Wretched is a good set, but one I can take or leave. There are other options. Mara, even just on the back bar. Trickery. Clever, for some. Even sets you may have never heard of in meta circles, such as Steadfast's Mettle. The latter helps create medium armor builds that have both low dodge roll cost and can turtle with low block cost.

To Balorgh and Bloodspawn I would add Selene and Zoal at the very least. Selene, because I see it. Zoal, because I love it myself. In terms of mythics, I think Markyn and Wild Hunt are still obvious options. I know what I'm saying doesn't add huge variety and most of it is meta, but these are examples I would personally use.

I like playing around with off-meta stuff, though. Maybe that doesn't make me a typical player, or at least not a competitive one (I am not). On the other hand there are IMO two components that need to come together, before something becomes meta: It has to be (marginally) OP or universally useful (Markyn, Rallying Cry) and it has to be promoted by someone with sufficient clout or otherwise go viral in the community. The two of those things do not necessarily go hand in hand. If you buildcraft yourself you may do well with off-meta setups. Not everyone talks about what they run. Perhaps more importantly people form habits. What works for them is usually coloured by their playstyle. Some diversity at least comes from that.

Not in PVP.

Not even close.

Agree with jtm^^

PVP is 90% meta-humpers due to no class balance or build diversity.