Would be fun. FFXIV is actually well suited towards it because it's easy to create and play multiple characters unlike RuneScape, where ironman mode is very popular nonetheless.
If you don't know what it is, it's just no reliance on other players when possible. So in the context of FFXIV, no trading or market board.
Think of it like purposefully opting into free trial restrictions on a character.
The idea is to create a more engaging playthrough where you have to rely on gathering, crafting, and NPC drops and vendors more to progress through the game.
Group ironman is the same thing but you opt into it with multiple people and you're specifically allowed to trade with each other but not anyone else.
As a result of market board restrictions, I would give out free retainers to ironman players for extra storage space. They need it because it's kind of important to hoard resources in ironman.