What would I like to have?
A checkbox I can enable when I start a Group search so I can lock out people who do not have a previous kill of a Coil Turn, Ex fight or whatever I am trying to get a group for (which is not supposed to be a training party but a kill/farm party).
This "win" information for each dungeon or Raid is stored on each character anyways so SE does not need to install new flags. Only a check that is done when somebody tries to enter the group through Group search and the checkbox "previous clearance required" or similiar was checked from the one who starts the search.
Why would I like to have it?
Too many players give a damn about it if you include in the group search description that a former kill or several ones are a prerequisite to enter the party.
Some solve this problem with loot links or a Link to Lodestone achievements but the latter is a no go for me on a PS4 because the link is really long to type.
Even if I could check out players on Lodestone more easily, I do not really feel to have to stalk them just in order to get an experienced party together.
I can't see it anymore when everybody claims to have downed it but then upon entering, you see that you receive a Myth bonus for at least one of the players in your group. And there are also the blatant ones who just hope they beat it faster if the rest is experienced at least. And they do not care that they are holding seven other players back. The fights are designed brutally enough that one weak spot can put the entire party on hold. Of course you can ask such players to leave or force kick them if needed but then you search again and I do not have unlimited time.
Not saying that everything is fine if somebody has beaten a fight once (he could have been carried through it, dead most of the time etc.) but it is at least an indicator that somebody has seen all mechanics in all phases at least. So please improve the group finder. It is a very nice tool already but it needs more tools to search for specific players.
TL/DR: Install a checkbox where you can start looking for a group of players with a previous win as mandarory to enter.