Dungeons. Trials. Battlegrounds. Cyrodiil. Imperial City. World Bosses. Incursions. The game is already full of group content, which promotes group play.They should promote group play anyway. Not everyone is a loner, and ESO is still partly an MMO. Perhaps give Quest Puzzles the ability to be solved solo, but grouping up makes it a bit easier?
For me, extra difficulty will make questing even more boring. It's not going to be more challenging, it will just be more tedious, since everything will take longer to kill. I lament that my main quester is my sorc and not my NB, because if they don't make extra difficulty into a toggle, I'd just rather skip spriggans that take twice the time to kill, with no extra mechanics, or anything to make it at least a bit interesting, fight-wise.