To do FC only housing, that is mediums and large only, they would have to make a specific zone I would think.
They COULD make it so a small house would be bigger on the inside, (aka: a house on the outside that could literally be the size of a... I dunno... police box? But full size or larger once you pass through the opening)
but that would also mean the outside furnishings would be limited. remember, small houses are limited to 1 garden patch and 20 items, including said garden patch.
Still... I think what could be offered, is that if you an FC using a house, no matter the size of the house, you can have the option of buying an 'indoor expansion', to accomidate things to enhance an FC's capabilities. Like the crafting tables and full set of merchants.

Truth be told, the whole housing situation needs to be drastically redone.
With the loss of Wildstar, and their, what was arguably, THE greatest housing content in ANY game, many other games, especially FF, need to step it up and try and match what these other companies strove to offer.