I know ARR was cut down but it really needs more done with it.

[Jeeqbit]

As a XIVDiscussion post brought it up, the game could use some new content added to Old Expacs in the way of side activities

The entire side activities from each expansion are available to them at each level cap (50, 60, 70 and 80). Which is a lot. Even at level 50 you have access to so much side content.

or opening deep dungeons at earlier levels

But you open POTD at level 17. It doesn't get much earlier than that.

Heck, even adding 1 or 2 new Gold Saucer fully developed minigames could go a long way to keep you engaged between MSQ sessions

This content does get added to. Doman Mahjong and open TT tournaments are the most recent, and a new GATE was added recently.