What are Icons? Icons are part of the 13th Age's "Archmage Engine" and I have discussed them previously in this blog. I can also point you to the SRD webpage or download the PDF direct from Pelgrane Press (also on DriveThruRPG).
The short version is that they are the big movers and shakers in a game setting. The folks who wield the most mortal influence on the world and can help to make thing happen, or prevent them from doing so. More importantly to your story though they act as touchstones for your characters, people for whom the PCs may be working for, or against, and can help generate good roleplay moments through the complications they provide.
How are Icons going to help spur cool roleplaying? Well when characters are created they get a certain number of relationship points to divide as they see fit among as many icons as they see fit (limited by their points). These can be positive, negative, or conflicted relationships, or a combination. Conflicted relationships are just that; sometimes these powerful figures will be one your side, and others not, and there may be some layer of complexity beyond simple enmity or allegiance.
Using three starting points you could have a character with a minor positive relationship with the Archmage (1), and a significantly negative relationship with the Warlord (2). At the start of a given session/adventure the players roll d6s for each point in each relationship on a 5 or 6 that relationship will come up during play.
So I roll a d6 for the Archmage and get a 4, and 2d6 for the Warlord and get a 2 and a 6. I keep the 6 and at some point during play I can "spend" that to gain some kind of meaningful advantage from that relationship. I may be able to spend that to gain help from others who oppose the Warlord as I do, or I may be able to use my enmity with the Warlord to intimidate some of his lackies. Had I rolled a 5 I would still be able to gain an advantage, but it would also cost me something, complicating the story in some way; perhaps the lackies return to the Warlord and now I have a war party coming to hunt me down!
Icons in Numenera
Having already gone into detail on what they are and why I think they are cool I'd like to propose some potential Icons for use in the Ninth World. These Icons are suggestions that I think would be an appropriate fit for the world of Numenera and who would make excellent allies and enemies for the players.
The Amber Pope
This one is probably the easiest and most obvious. The Amber Pop reigns over the Order of Truth in the Steadfast and by extension has power over almost all of the region's major governments, organizations, and monarchs. The Pope is motivated to lift humanity out of its current dark(ish) age by reclaiming the numenera for use. That the Order of Truth is beginning to steer its followers toward a religious bent as a means to do so may not go over well for everybody however.
The Grand Magister
If the Order of Truth seeks understanding of the numenera for the betterment of all mankind, the Convergence would be its direct opposite with members seeking the same knowledge and understanding as a means of power and control. Therefore the Grand Magister would be the Amber Pope's opposite, with only loose control and personal gain in mind. Where the Order is vast and numerous the membership of the Convergence is small and elite, but no less powerful.
The Lucid Dreamseeker
The Jagged Dream seeks to see the world burn with conflict, and they have spread like a cancer throughout the world. Though the cult operates in a cell structure, there is one dreamseeker who secretly controls all, pulling the strings of war and conflict with one hand, while using the other to forge great nations and powerful groups for latter use. The goals of the Jagged Dream oppose those of the Order of Truth, but do not always interfere with the Convergence and at times the two might align, though seldom in a way that both parties would realize.
The Bandit Queen
The woman known as Dissicaeli (Numenera Bestiary pg 146) currently holds this position. She is a warlord like no other with a thirst for personal power and wealth. Her tactics involve raiding and pillaging those who have to amass her fortunes. Luckily for the Steadfast she has contained herself to the Pytharon Empire thus far, but for a woman as powerful as her it is only a matter of time before she sets sights on something or somewhere greater.
This Icon assumes a slight alteration to the Angulan Knights as written; instead of being a decentralized fraternity (though they are not male exclusive that term just feels like a good fit) of knights the is in fact a single knight chosen to assume the role of the Paladin. This man or woman is chosen in secret only by the Grand Knights. It is the Paladin who make policy and guides the Angulan Knights with a subtle hand toward whatever goals the original Angule laid down in secret some 350 years ago.
While mutants in the Ninth World don't have their own kingdom or society, or even a unified philosophy there has always been an Evolutionary. One mutant who above all others stands for the rights of their kind against whatever oppression may come. The current Evolutionary is a woman named Mila the Mindslayer (Numenera Bestiary pg 154). She has taken it upon herself to wage a war against the Angulan Knights above all others, setting herself on a collision course with the Paladin.
These are just a few ideas that jump out at me as being cool ways to introduce additional layers of world detail and allow the players to have their hands in some of the larger scale machinations within the Steadfast. 13th Age has thirteen icons to work with, and they each have their own conflicts and alliances with the other icons. If one were inclined toward using Icons in Numenera there is nothing to stop you from creating only six, ten, or even twenty icons as you saw fit. You could broaden your scope the the world in total, or keep it narrowed to a small section.
Hopefully this will give you some ideas to potentially consider using Icons in Numenera, or maybe in some other game. I think the core concept is mechanically light enough, and robust enough in its roleplay elements that it can fit into just about any kind of game within any setting.