So, here are a few specific tricks you might want to consider to build an interesting Icon set:
The Icon Deck
One interesting thing about the existing 13 Icons is that they're not hard to map to half of the greater arcana of the Tarot deck. Some of that is probably intentional, but it's also almost inescapable because fortune-telling and iconic characters use the same kind of broad, recognizable strokes that it's easy to map from one to the other.
So, given that, pick the method of your choice: Tarot Cards, Viking Runes, the I-Ching - whatever floats your boat, and assign the existing Icons to it. Then, look at the unassigned elements, and start creating Icons based on those, using some of the guidelines I talked about yesterday (specifically, put some thought into the places each one suggests). I like Tarot for this, but that's just me.
When you're done, you'll have more Icons than you can use, and that's great. Now pick 13 of them at random, and figure out what kind of world that creates. Or if you want some collaboration, have each player pick one, then select the rest at random. The idea is to create unexpected combinations and see if they ignite the spark of creation.
One of the first things that struck me about the Icons model was how easily it mapped to the Amber DRPG. For the unfamiliar, characters in Amber are the children of the Princes and Princesses of Amber, all of whom are potent, iconic characters in their own right. Amber is very nearly the definitive "characters as setting" game, since almost everything else outside the characters is subject to redefinition. Now, while a lot of the strength of Zelazny's character's came from his ability to sketch with a few bold strokes, there's nothing that says you can't attempt to steal his thunder a bit. A game where the Icons are a fairly tightly knit group (like a family) can have a really strong interpersonal dynamic.
Curiously, with just a small tweak, this applies just as easily to supers. It would not be hard to pick 13 Icons from DC or Marvel and use them as the basis for a game. Obviously, which 13 you pick will say a lot about your game, but that's half the fun of it.
The Old Ones
When 3e came out, White Wolf released a very interesting setting called The Scarred Lands. One of its core conceits was that the defeat of the Titans by the gods was not terribly long ago, and the dead or bound titans cast a huge shadow across the setting. For example, one's heart had been ripped out and he'd been chained at the bottom of the sea. Because the wound bled constantly, that whole area of ocean was red with it, and tainted with his essence.
If you were to model this with Icons, you would have a very interesting arrangement because these Icons would explicitly not be active, but they would still have factions surrounding them (lingering worshippers, those tainted and so on). What's more, it would be a very lopsided set of icons, since the real story is how the world manages to move forward while still bearing the burden of these things. By making them the icons (rather than making them SOME of the icons, but also putting icons at odds with them) then you make them central to play, but often in an unwelcome way.
Turned up a notch or two, this is also a great way to use Icons in a horror context. If all the Icons in your game are Great Old Ones or Lords of Ravenloft, then it's pretty clear the odds are stacked way the hell against you. The bulk of play is in opposition to these forces, but the sheer scope of them means that maybe it's worth risking a complicated (or even positive) relationship with a perceived "lesser evil" in order to fight on.
Suppose not all the Icons are known to the players at the outset of the game. They will be revealed over time and as a result of events in play, and it's expected that player's relationships will change or evolve over time. This is particularly useful for two sorts of game.
First, this is a great way to model a world in flux, such as is the case in the classic Dragonlance adventures. The existing Icons are in a rough sort of stasis, but the introduction of new Icons throws everything into disarray.
Second, it's a good way to model characters whose eyes are opened to a deeper, secret world, as is appropriate for many conspiracy or espionage focused games.
It should go without saying, but in both of these cases, players should know what's up. Even if they don't know what new Icons wil be revealed (though many players can handled that out of character knowledge quite well) they should know what the GM has planned in broad strokes. You're trying to make for a cool experience, not trick the players.
One other subtrick for this - you can occasionally use a proxy icon when you have a mystery threat, where the apparent Icon is actually the servant of the Real Icon. If one Icon is the main villain or threat of your game, this basically allows you to treat that icon as a procession of bosses, which might be cool, especially if you're feeling kind of video-gamey. And speaking of which...
So, I was thinking about Mass Effect, and what Icons I would use for that. A few obvious ones jumped out - Harbinger, Hacket, The Shadow Broker, The Illusive Man, The Council and such, but I eventually petered out. And that lead to the weird realization that I would probably round out the list with Garrus, Liara, Tali, Wrex and the others, and that suggested something weird.
It's almost an inversion of the Old Ones model, but imagine if the Icons (or more aptly, Anchors) for your game were those people closest to you. It upends the relationship, putting the hero in the primary position, and making the Anchors primarily responsive/reactive. It's a really weird idea, unless you've played any Bioware RPGs, in which case you're pretty comfortable with the idea that your relationships with the NPCs around you is one of the major avenues of play.
So, yeah, this is kind of a weird, one, and maybe not a great match, but I want to call it out because it has me thinking.
I may have one more 13th Age post in me. We'll see after I've gotten some sleep.
1 - If you have a real dedication to this, using an Everway fortune deck would be three kinds of awesomesauce.