Maybe we could use guildmarks towards the purchase of our player housing. Like the guildmarks are a 1:1 ratio for the purchase, but once you're out of marks you have to pay the remainder of the price in gil.

And/or Guildmarks could buy furnishing etc. for your home.

That being said: I'd like the gil. :P

Also, you can already get all your guildmarks you spent on traits back. Just go to each guild and talk to the NPC. They give you back your spent marks.