So besides the obvious choices (and some might prefer them, so if you do speak up, because this is the place for it) :

Convert Guild Marks into Gil (more than likely it will be something like 1 Gil per 3-5 Marks conversion) [That's 16,667-10,000 Gil for 50,000 Marks for reference]

Convert Guild Marks into Grand Company Seals (again will probably be a 1 Seal per 50-100 Marks) [That's 250-500 Seals for 50,000 Marks for the less savvy mathematicians]

Can we think of anything else to do with them? I feel they add an interesting and different element to the leves/solo-low man play of the game. So here is an idea I was cooking up:

First off, scrap Factions (Horn and Hand, Broken Blade, Azyema's Shield) they are seemingly random anyway, and with the incorporation of Grand Companies taking the main stage it seems odd for these "factions" that don't really play a big ol'part in the world. (at least I have yet to really see much reference to them in the game, and I've done all the quests/story lines)

So get rid of those, now we don't have Faction to build up, so how can we get those special Faction Leves? Rename those leves to Special Operations/Elite Leves/Hunter Leves/what ever you wish to call them, name isn't too particular. Once renamed have certain leves give out the Marks instead of Gil like same (instead rename the "Class_Name Guild Marks" to "City_Name Leve Marks") Each of the three cities will then have their own set of "Elite Leves" wherein you can get those same Faction Leves completed.

Now you might be saying "This sounds redundant on itself." And from a veteran player like yourself, I would agree. But we have to think of the new players as well, and to be honest having Factions that contest for ranks at the Aetherytes as well as having Grand Companies contesting for power, and they never speak of each other...seems a bit odd and could be confusing to a new player.

Also what this does is it allows for Grand Companies to talk full control over the land that is about to go into a state of War and removes the "R&D" Factions from the front lines. Also another point is it could potentially make the "Faction Leves" now renamed, have a higher frequency of use, assuming you could build up your points faster.

Also since you begin getting Marks at an early level why not have a person who can exchange those Marks for healing potions/free chocobo rental coupons/dyes/small stuff like (I would suggest NO ARMOR in this list, as that would be the trade off; either spend the points on the Elite Leve and get the armor or spend the points to get healing items to help progress easier).

Okay long post and I swear I'm just about finished, I would just like to open the floor up to any and all to either discuss this proposed idea or to simply post your own idea. Get those creative juices flowing ladies and lads!