IMO,
"All content" should be available to all players regardless of skill or difficulty, solo or 24-man parties. That said,
The game really needs to change how things are gated, because I think attracting new players to the game is pretty much done. Nobody really wants to get into a MMORPG that they haven't a chance in hell of catching up on.
The squadrons and POTD are a good way for DPS to level another job, not play the game. These are actually pretty tedious and boring, if not broken ways of leveling another job. But it does bring up an interesting detail, they could be used to bypass the storyline dungeons that they can't get anyone to queue for, or are too bored of (before level 50, you will keep getting Satasha and Tam-Tara Deepcroft, over and over and over in the leveling queue, no joke.)
The "old" harder content, eg Coil, and even Alexander, should be content that players can queue for, and if there are no players available, switch to AI players and maybe beef up the AI squadron players to compensate for their inability to do the mechanics well.
In other games, typically there is no actual gate to ENTER the harder content, but you can also just pot-spam your way through a lot of content with enough money, which IMO is not fun at all, essentially funding becomes the gate instead of skill. In other (less fun) games, there is no level sync, so you can only play a dungeon until you can beat it naked (no gear equipped) and then it's pointless to ever play again.
I don't want to see "difficulty sliders" become a thing in FFXIV, because that is extreme laziness on the part of the developer to artificially inflate the difficulty, without making any actual change to the content.
But I actually want a few "long" dungeons that have puzzles to them, where you can hit a checkpoint in the dungeon and exit and come back later with help, or your squadron, rather than have to camp in the dungeon until someone comes along to help. Puzzles are unfortunately spoiled by internet guides/videos, so something would have to be clever and personalized to really do this.