Member reqs for FC houses will result in people being spammed invites even more than they currently are and dead accounts being kept in FC's. An FC with a large member count does NOT mean it is active. I have been in an FC with hundreds of members before and 90% of them were inactive. After that jebait I was sure that the next FC I joined was actually full of active players.

A small FC run by RPers can serve more people than a large FC full of dead members.

Other people might suggest a harsher minimum activity per month, this only serves to punish more casual FC's.

Increase the supply of houses vastly, FFXIV makes them so much money but they don't want to invest in expanding houses to serve a majority of their playerbase. I would be happy with one instanced ward per housing area, you can get your dream plot and can bump into plenty other people all living around you.