Similar to the FF11 model, Adventure follower, retainers should be able to accompany you on your journey to assist you in open world content. While we have the Chocoba system that already fills that purpose, it lacks depth. The Chocoba system does not have an advance class system, leveling, or customization outside of Barding or color dye. While the Devs can focus on the Chocoba system and deliver these demands, I and perhaps other players would prefer to have a more humanoid companion.
So what can they or should they do?
Well I think companions overall should have the same limitations of Chocobas, such as being dismiss in cities, housing, dungeons, and take up slots in parties when active. Only combat classes should be allowed to venture out with the player and would require a venture coin or similar fashion to summon them for a limited time. They should be able to level through combat and ventures, learning new abilities similar to that of a normal player. They should not be stronger than a normal player, but strong enough to feel that they are contributing in a fight.
Story:
Besides combat, they should have their own story based off their race, sex, or class. These stories should be a simple side quest(s) giving the retainers depth.
Balance:
When it comes gear and class combinations some could argue a balancing issue with the retainers, but due to them being open world it should not be too much of an issue to the player base. My idea is for them to only have a ilevel power. Such as having a ilevel of (X) would only net them a (%) base damage, heal, or HP.
Leveling system:
Retainers should be able to level with the player and through ventures. They should sync with the player level so long as they are higher level than the player. If the player is higher level than the retainer will be summoned with its current level.
Unlocking:
I put a lot of thought into it and ultimately believe that lvl 50 patch 2.1 content is the best time to unlock this content. Retainer would be unlock through side quest(s) and required to be atleast level 30 to unlock. most job abilities will be avaliable at level and This allows the Chocoba system to still be used. This also establishes the player as the warrior of light instead of an adventurer who was Shanghai into killing primals.
Armor/ weapon:
Same limitation that your retainer currently has. If you want to fight your way through the world with Lord Hein or a katana wielding school girl by all means!
TLR
Retainers should be your companion through the world leveling with you as you quest, hunt, or complete fates.