3rd Retainer by Default, please?

[Ophie-Mio]

I don't care how, but there MUST be more inventory space added on either a character or house.

With current amounts of retainers its quite tough but manageable, but 3 even more classes with its own armor and weapon glamour, 10 new levels for old jobs, and new tier of materials and materias it will destroy the current amount of space.

Exactly. This right here. We don't care where, we just want storage!
However, with the addition of three more jobs, those weapons and glamours, 10 new levels to achieve for your old jobs, all that gear...
Plus, if you're a crafter, all THAT whooha will wreck a normal player.