Asking for Gil for marks is just going to lead to disappointment, either they will give you 1gil per mark in which case most people will complain but still give most people that did leves every day +500k or if they give more than 1gil per mark it will mess up the economy even more. The marks from crafting classes should be used like they where in FF11 to buy crystals for signing gear (this would be a great thing to implement), crafting + gear (such as apron) and in house items that gave you a boost for that specific craft. The last 2 items mention were high guild point cost which took months to achieve and normally quite a bit of gil in items to be handed in but the rewards where worth it.

A new suggestion would be using the marks at any repair NPC to gain access to master support, this would of course be dependant on them added guild marks back into the game.

As for guild marks from battle and gathering not sure there is really anything they can add to make it seem worth while and at the same time not really game breaking. Their best option would be to convert the marks into GC marks there are at least some items there that people can get.