what kind of content do you think they could add as expansions? new lands and races?

new classes probably.

or maybe new skill line for classes

necromancy is still not really used

archery overall is not really being used (the bow atm is just the ranged filling slot for stamina based characters)

more lands obviously as you can see by opening the map, and this was confirmed by them.

as for the rest is just speculation, more classes probably, but most likely more skill line for existing characters is my bet.

more guilds obviously, a law enforcement system, maybe killable npc's, who knows.

New skills - There's a wealth of skills that we're used to in Elder Scrolls games that haven't been touched on in ESO yet.

New Classes - Each game until Skyrim used classes (though you were always free to make your own) and as a result there's around 25 different classes only 4 of which are in ESO.

New Guilds - Thieves Guild and Dark Brotherhood for certain, maybe some variation of The Blades or the Imperial Legion (trying to work to restore the Empire?), maybe a daedric cult for PCs to join?

New areas - Obviously. There's a huge world out there.

Arenas - I think it's just a matter of time for these.

New Races - That's really iffy. Most of the races that have ever been PC viable are accounted for. So this leaves races that have only ever been NPC. Falmer? Akavir? Tsaesci? I'm doubtful we'll ever see more PC races, unless they put together an entirely new Faction with 3 more races.

Typical stuff classes zones skills story

No more races, plz and thx

You don't want the all new zombie imperial class, now with less bones!

Only if they can't run

No new races breaks lore

Probably the other continent, Akavir.

They will have to finish this continent first ...