Many role-players already design their Midlanders and Highlanders to be 'Garleans' to a pretty satisfying effect. Simply adding a 'third eye' glamour that can be dyed would go a long way towards appeasing those individuals.

With that said, I adore Garleans and find them to be the most interesting race within FFXIV's setting. I wouldn't be opposed to them being added as a playable race at some point but at the same time...it'd need to be done exceptionally well and I don't believe that it should be done until the customisation options for the existing playable races have been expanded.

As for the Garlean's inability to wield magic is concerned? The lore book states that some Garleans are capable of using it - it wouldn't be unreasonable to handwave it with the Echo or a certain plot element introduced in 4.0 that I won't mention to avoid spoilers.