Expansion wise, I would love if there's heavy content that's lore-based. Specifically more information/activities with the other 5 remaining Elder Dragons (formal reveal of the underwater dragon or the 6th dragon) would be awesome.
They will not be able to initiate the explore mode but they can still go on explore mission if somebody else in the party starts it. A character that has not completed the story mode cannot start the explore mission but they can join if someone else in the party has.
I do not think that Ascended armors sets(if they do come) will be that much of an issue.
If the screenshot and the information provided in the new post is true then they will lack upgrade slots. Without the additional stats provided from rune upgrades the Ascended will ultimately be weaker despite their initial higher stats compared to exotic equips. The problem lies in rather or not these Infusion upgrades will provide additional stats like those of runes or just simply resistance against Agony or future type of monsters/conditions/etc.