A couple suggestions...

First idea: You could add in an "Order" function that would allow players to "Place an order" for a certain item, lets say Wind Shards, choose their buying price, say... 80gil each, and let players choose if they want to fill the order or not. This would not only offer an instant sale option for people looking to sell their wares, but I believe it would help stabilize the extreme fluctuation of certain item prices since people only looking for a quick sale will not be undercutting the hell out of people on the MB just so they can sell quickly. This feature would require the buyer to have enough gil on hand at the time of the order to pay for the entire order and the gil would go into an escrow account of sorts until the order is fulfilled or cancelled. Having a time limit on the order of say 24-72 hours would allow people that want an item, but don't need it immediately, to go on about their business without having to scour the market board constantly to see if someone has listed the item they are looking for.You could even add in a small fee if someone places an order but decides to cancel it prematurely. This would help prevent people being stupid with the system and would discourage undercutting in the order system since they would incur a penalty for early removal. All in all, I think it would be a wonderful addition to the game and would help stabilize the economy a little bit... and with that penalty for early removal, you have another gil sink... which we all know the Devs LOVE!

Second idea: With the advent of personal housing, I would love to see the ability for friends to be able to teleport to my house. This could be done by selling or crafting some form of Aetheryte Coordinate voucher that could be given to friends. When someone buys/crafts said voucher, they could attune it to their house, and then give them to a friend. This would allow the friend to learn the coordinates for their house, and add it to their list of teleports. So as to not clutter up the teleport list, you could add a new category in the sorting tab that you implemented in 2.38 and make it customizable so that people can remove a coordinate if they no longer need it or want it. This would allow friends to come visit without having to go through the major cities or zone in through the wilds to get to someones house.

Just a couple things that were floating around in my brain yesterday while customizing my new house. :)...