Pretty much everything I suggested ages ago came true and I must say I really like the One Tamriel update. You're no longer restricted to any special end game areas, and there is a purpose to revisit "old" areas. The only thing I'm missing at the moment is a bit more challenge... Having 420CP character venturing in one tamriel is still a bit too easy. (I'm sure they have to balance it to fit a "casual" player) so my suggestions here could probably accommodate both, casual, and hardcore PvE players.
* Veteran Solo Content. Veteran Group Dungeons are nice, they provide a good challenge for groups, but regular solo content is way too easy. Currently I'm doing Non-Vet group dungeons and Undaunted Pledges, and Craglorn group dailies, solo because regular solo content (and public dungeons) is too easy. So one suggestions would be to have Vet mode for more solo content as well, as Maelstorm Arena to provide more challenges and greater rewards for the moments you don't feel like grouping. Maybe just a simple toggle would do the trick. Switch veteran mode on, and enemies has a bit more Health or hit a bit harder, for a bit beeter loot and more Experience points. Maybe with vet mode on, you could get a penalty to your resistances? Double or tripple the damage you take from regular content, so that you would be forced to use more trickery to survive. Currently most solo content is doable by just jumping in and spamming your favorite DPS skill, because nothing really hurts.
* Incremental content I would love to see some instanced solo delves, that opens up the next difficulty level every time you complete them. After completion, increase the difficulty a bit each time. Maybe perfect as a DLC? I wouldn't mind to buy a DLC, that provides some good REALLY CHALLENGING repeatable solo content with nice rewards.
* Duo Content Sometimes grouping is fun, but personally i prefer to duo with friends. Some content tailored for a duo setup would be nice, where roles don't matter that much. The content would be quite challenging for a duo setup, but wouldn't require any particular roles, like tanks, dps, healers.
Thanks for a great update with One Tamriel, and it was nice coming back after a few months off ESO.