Duo stuff. Wife and I love playing ESO together, but often what we need to do takes us to solo routes.
"Class identity isn’t just about power or efficiency. It’s about symbolic clarity, mechanical cohesion, and a shared visual and tactical language between players." - sans-culottes
I don't really have a number in mind - what I want most is a new Class. Especially now that Subclassing means that those of us who want to see the world with fresh eyes and a brand new set of skills (remember they changed the tutorial and redid the starter areas) can, but the people who don't want to have to level another character can still make use of the new stuff via Subclassing one of those lines in.
Subclassing also makes me think a new solo arena would be a fun thing, especially since Subclassing really encourages the idea of dropping your support lines (at least for the 3/7 Classes that have defined lines...) to min-max damage while depending on the group for support. A solo arena that requires players to be able to support themselves could make it so you can't just pull your shiny new Subclassed Arcplarblade through because you'd have to start thinking about needing to heal.
I do love dungeons, and I'm so happy that we're back to 4 dungeons this year instead of just 2 like we had the past two years. I'm already ready to get the next ones, and Finn's team has been cooking, as the kids say. And especially now that they're officially taking Balfiera away, that landmass is just sitting there waiting to be used...
But the one thing that ESO doesn't have that may be cool is a specifically duo arena. We haven't gotten an arena since Vateshran (and we haven't gotten a group arena since Blackrose), and even then people could go with Companions. Plus, the Arena Weapon sets still have one notable skill that never got a set to improve it - the ultimate.
As for a location, the Battlespire has been referenced a few times and it's something that'd make for a nice lore connection.