Overall, I like new endgame activities. Especially arena type ones, Onslaught and Headhunt, are my favorite. They are simple, short, and fun. Sabotage is less favorite, since it takes more time, and I generally don't like anything invulnerable (includes Aurora Knights), but still fun.

Mandate
Mandate is now a part of endgame rotation. And they are not good. Mandate is somewhat pointless. Just roaming and killing in a empty map is not quite fun. Also it takes some time to finish, but does not provide good enough reward for the effort. I think Mandate could be improved by adding...

A short Hunt with smaller reward than the Expedition Hunt.

Secondary objectives, such as "killing x champions", "destroy x Haunt Stones", "clear a Secondary Area", which reward with items and gold upon completion.

Or anything to do that reward the effort.

Secondary Area
Secondary Areas are not interesting. I like them, but rarely delve into them because they don't give any meaningful reward.
Adding a small objective, such as a boss fight, at the end of the Area that rewards with gems (with small possibility of Genesis Stone) would make Secondary Areas more interesting, since gems are crucial items in crafting.
Also, place Hunt trails in Secondary Areas might be a good idea to give them meaning, though it might be bad because some players would find them just a chore. I don't know. I like small delves.

Cosmetics
Cosmetic items in Mirrormaid Ship should be drop items (like dyes), or should be bought with a special cosmetic currency (like Enneract and Primordial Affinity), to provide players something to hunt. Just paying 100000 gold is not quite rewarding experience.