I would like to see scalable dungeons so they can be done solo if needed but offering lesser rewards.

If not, then I do think the requirement to do them in order to unlock things like Retainers or more of the storyline should be removed entirely.

SWtoR came up with something of a solution to this problem when they introduced 'Tactical Flashpoints'. Basically dungeons which can be done with any combination of classes, even with all DPS in the party. Healing stations are located at boss fights and the content scales in difficulty depending on the group make up. I doubt such an overhaul could be done to FFXIV but I'm not a developer.