Some of this idea may already exists, but I'm sure the ffxiah crew could do a much nicer & online version of it.
I've been thinking that is would be really nice to have a interface that let you build and equip your character with any item (appropriate to the job / level of course).
So some kind of screen where you pick a race, job (with level), subjob (with level) and then you can equip it. Having the race and jobs you can determine the base stats. Maybe add a way to add adjustment for merits. Each time you put in a pice of gear the stats for the char should update. Would also be good to have additional info displayed like how much +acc, +racc, +mab you are wearing (all the important stats the game doesn't tell yo). Maybe even have a feature that generates the /equip <slot> <item> text so you can paste it into a macro.
I persoanlly think this would be useful for setting up gear paths, customizing gear sets for different situations (weapon skill, Tp ect..) even just keeping track of how much MAB you are wearing without having to add it up (lazy I know)