One of the main elements of the Planescape setting is (funnily enough) the Planes. It's also well known that they can influence people, and change them in unexpected ways, or otherwise suffuse their substance into a living being. These beings are referred to as the Planetouched, and are a fairly common sight on the streets of Sigil.
Touched (possibly innapropriately) by the Inner Planes, the Genasi have the power of raw elements flowing through their blood.
Fire Genasi are your bog-standard "man with a flaming head". Fairly uninspiring, I feel. They're quick to anger, passionate, etc. So why not mix it up a little and have one who is more about cold fury and rage, less a naked flame and more a blue-hot flamethrower? They also have some minor fire-based abilities (like fire-affecting, or even a version of the Burning Hands spell) - in Risus, you can stretch these to be as varied as you want (say, you can affect fires, or shoot fire, or walk unharmed through an inferno) - whatever you and the GM agree is appropriate (no, there isn't a "Nuclear Holocaust" setting on your personal flamethrower!). Good for hot-blooded Warrior characters, and can make some very thematically-focused Fire Wizards (though something like Sorcerer, with the whole "magic in the blood" feel might be more appropriate
Water Genasi are also a bit hard to pin down - appropriately enough, for an element which flows and adapts. Not much exciting here, sadly. I tend to see them as just as passionate as Fire, slightly tempered by a nicer nature. But when the chips are down, you could face a torrent of pain. "Flexible" would be a good word for them - in fact, in the standard D&D race lineup, where Dwarves are Earth-like, Elves are Air, etc. I see humanity as closest to Water - not just adaptable and mutable, but prone to quick change, and never safe to predict. Awesome Monks, fair warriors, might make an interesting Wizard...
Air are a personal favourite - they are flighty, artistic, philosophical... another recurring NPC of mine is Sadia Windstrider, a warrior-poet with an acrobatic combat style. Most of the cliches you would use for Elves are a great fit for the Wind Dukes. Good as more skill-based characters, rather than straight up Warriors - thieves, Bards, maybe the odd Cleric.
Earth Genasi are big, ponderous, and stoic. Again, pretty much normal-height Dwarves - they make good Warriors and Defenders, being tough as nails and strong as oxen. They would also make for interesting lawyers - a well-thought defense, which might take hours to formulate, and an unshakeable nature make them surprisingly handy in planned-out social situations.
The Good, The Bad, and The Not-So Ugly
Then you have Tieflings and Aasimar - descendants of Devils/Demons and Angels, respectively. They are very generic in their stance - Aasimar are happy, beautiful, shiny people, and Tieflings are sullen, cloven-footed and horned folk. I think I'd keep these generic models as distant relatives of other Celestial/Fiendish entities - so, a Half-Fiends great great grandson, for example. I'd much rather see some variety based on the particular Planar Being in the bloodline - so, you have Planetar Aasimar, with a slight greenish hue and a warrior bent, or an Ice Devil Tiefling, with blueish skin and odd, insectoid features. Of course, each of these would require a lot of conversoin work and balancing to be done in D&D - but one of the joys of Risus is that they would be as useful as you wanted (having Planetar Aasimar as a cliche might give you an advantage when talking to Celestials or superstitious common folk, make you a kind and gentle person or a righteous holy warrior, give you a commonly-attributed ability of Planetars - but for the love of the Gods, not Gate).
Pretty much pick an interesting Celestial or Fiend, and run through what you want out of the deal with the GM. Simple.
Lawful, Neutral, And Chaotic Planetouched
So, here you have the Zenthyri (beings descended from pure Law), who are surprisingly interesting - they are Lawful to a tee, and might seem aloof and uninterested to most. But they are simply very good at controlling their emotions, to the point that, when faced in battle, they maintain a "tranquil fury" that is terrifying. They also give some great roleplay options - while they are a natural fit as Guvnors, imagine a Zenthyri Sensate - wanting to experience everything, and recording and studying each sensation, making notes and comparisons.
"Well, that Burning Hands spell did hurt, but it was nothing like the Fireball last week... I'd say a 3.8 pain rating, as opposed to the 7.9 Fireball..."
You ever wanted to play a blue Vulcan, they're your guys.
Chaonds are those poor sods touched by Chaos - misshapen, dour, and altogether mad. Very useful as NPCs, they make for great not-evil bad guys, or as suspicious characters who turn out to be not as evil as they seem... as characters, they sadly have a tendancy to be picked by what the internet terms as the "lolrandom" player - the guy who takes Chaotic Neutral as an alignment, then makes a character who could kiss you or punch your baby - you'll never know till it's too late. Can be good, but I'd keep an eye on it...
But then, we have the Rilmani. Those touched by Neutrality.
Sadly, yes, they look pretty much like these guys - faces like runny cheese, no specific tone of voice or action... Seriously, why bother? Again, much better as NPCs, maybe as perfectly neutral judges in contests or, in the rare case of a Lawful Neutral one, a perfect Guvnor to track down your player's resident lawbreakers. Really, dull enough that they kinda slip into the background (and as such, perfect NPCs!).
Next, the Para- and Quasi-Elemental Genasi - Obscure and Weird, here we come!