I'd like to "bump" this thread as I think that this is one of the more underused features in the game that could provide a lot of assistance in parties in-game. I like a lot of the ideas that are presented here and I have a few of my own to present as well. Some of my ideas run parallel to the ones other people have stated here also, but I'll still present them for ease of reference.
Firstly, I agree that all of the Ally/Enemy markers should be allowed for use in any circumstances, not just in Leve-quests. Let us use the markers whenever we feel, please.
Next, as DoctorMog stated before, take away the mandatory descriptions in the sign lists that state what each sign should be used for. If anything, list the signs in the same manner as the Macro picture menu. Just let us choose a sign, choose the target, and be off with our adventure. The party members, I'm sure, could attempt to dictate what certain signs mean to themselves. Or, if anything, the signs will be universal enough to understand simply at a glance.
Another thing is to add more relevant signs that can be used in multiple situations (unlike the card suits already listed). Some examples are as follows:
1.) Numbers signs - ( 1, 2, 3, 4, 5, 6, 7, and 8): For the sake of numbering enemies for a bunch of miscellaneous reasons. Numbers up to 8 seems a bit excessive, but it would be enough to cover a wide variety of uses.
2.) Status Effects signs - (Bind, Sleep, Paralyze, Heavy, Slow, etc.): For the use of letting party members with the abilities to inflict such status effects know exactly which enemy to target and what move needs to be done on them. For instance, in the Moogle Primal fight, it's helpful to keep Kupdi Koop (GLA) Moogle bound so that he's not harassing healers and DDs during the fight. With a "Bind" enemy sign, if the Archers in the party see Kupdi Koop floating about flailing away at the Conjurers, they'll know that he's supposed to be bound and can readily bind him on the spot if he's moving about and has the respective icon on him.
3.) Attack / Do not attack signs: These will better coordinate the party's attack methods during battles of any kind if people know which enemies to attack and which ones not to attack. These are already in the list; however, being able to use them freely would be really appreciated.
4.) Miscellaneous Enemy signs (Exclamation Mark, a Kite, Skulls, Fire, Ice, Wind, Thunder, Stone, etc): Of course, Misc signs are helpful too for other situations. One such example being the Kite, it's sort of self explanatory. Players could put kites on the enemies that they would like to be given the "run around" so that other players can focus their attacks on another monster (like one marked with an exclamation mark or a number).
More Ally signs would be useful as well. Perhaps a Shield designating the "Tank" of the group and another type of sign (Perhaps a Red Cross or Medical sign) designating the "healers" of the group. Keeping track of these two types of roles in the game with signs would make things a bit easier in knowing who's who and how to properly form a strategy to keep these people from being defeated in the battle.
These are some of the ideas I had jingling around in my mind. Hopefully they help.