D&D is a game about exploration with no mechanics for exploration, just success (XP, treasure) or failure (traps, combat, hireling loyalty).
Taken from that perspective, what mechanics would you want in a game about social interaction?
It's an intriguing question and feeds directly into some of my personal interests. I also don't currently have the time and discipline to compose a Twitter-appropriate answer. So y'all get a blog post.
I'm going to talk about two sorts of mechanics here: randomized and inventory mechanics. D&D uses both, as do most of the games that came after. Randomized mechanics are the ones most folks think of when they discuss mechanics specifically, as opposed to rules. They're good when you want to introduce risk and uncertainty. Success usually yields a small reward (in classic D&D it opens up new areas for exploration and grants a few drips of treasure and EXP), but even success drains resources.
These resources are the basis for inventory mechanics. They basically are anything the party keeps track of that can be spent in the course of engaging in the game's primary activity. In classic D&D, we're talking about things like rations, light sources, rope, spells, and potions. Combined with the threat of combat and other randomized mechanics that threaten uncertain levels of resource drain, they serve as the (usually) soft limits on the amount of exploring the PCs can do. The game largely consists of the players judging the amount of risk vs. reward on further exploration based on the amount of resources they have left.
So, in a game of social interaction, we need rewards for successful interactions, and if we use classic D&D as our model, resources that are spent in social interaction and randomized events which could unexpected cause a greater drain on these resources.
First, let's take a look at the rewards. One of the interesting things about the rewards in classic D&D is that they primarily allow for more exploration. Higher levels, more magic, and large piles of coins give the PCs greater resources, which allow them to dare bigger risks. This creates an interesting feedback loop where the players are not only encouraged by success to do more exploring, but also given the wherewithal to do that exploring.
Taking this to our social interaction RPG, we can easily port over money as a reward for social interaction. It might be hard currency, as in D&D, or goods offered by clients who need our PCs to intercede on their behalf. (And these clients could also spur the game as a potential source of “adventures.”) On the other hand, the “currency” might be something more like credit. The more well-known you become, the more likely people are to loan you things, loan you money, or even give you things or do things for you for free, simply so they can get exposure to a larger pool of potential customers and have their work associated with you.
Experience levels also map over quite well. If the culture of the game includes hierarchical levels (and most do), going up a level might correspond to achieving a level of social fame that allows you to elevate yourself to the next hierarchical category: from commoner to noble, from layperson to clergy, from apprentice to master. Cultures with varied layers of social hierarchy and interaction would lend themselves well to this sort of gaming.
So these are your rewards. But how do you earn them? By engaging in social activities and interactions. Some of these may require very little risk for the PCs, but the greatest rewards should go to those who dare the most or find clever ways to minimize their risks, just as in classic D&D. What sort of activities are we talking about? I imagine they might be things like putting on events and spectacles (parties, circuses, the military defeat of a hated or threatening enemy, the publication of memoirs), attempts to forge personal alliances (marriages, blood-brotherhood, partnerships, friendships, romances, treaties, commerce), and overcoming social rivals or triumphing in social contests (court battles, legislation, duels, arguing policy points before a voting assembly or a monarch).
And these offer other suggestions for more personal, but probably more potent rewards: reputation, trust, and respect. Risking these, along with personal fortune, social standing and perhaps even life and limb (“...we mutually pledge to each other our Lives, our Fortunes, and our sacred Honor.”), the players would engage in acts designed to increase their social standing and collective influence. There may be some risk involved (Are the barbarians crushed before they breach the gates? Is the gala rained out? Does the Duchess attend our party or go to the opera instead?) and dice may be rolled. But all of these only set the stage for what such a game would be about: taking on a persona and interacting with each other and the NPCs. Just as classic D&D was about exploring but had no rules about exploring, so this social game would leave the actual interactions up to the players of the game. How the players approached the NPCs, engaged with them, and created the web of relationships the game is about should be left to them and their GM, not the whim of dice.
This, honestly, is what I think should be at the core of a game based on properties like Babylon 5. As for Oddysey's game, I suspect she wants a bit more swashbuckling and derring-do than I've implied above, and a bit less high society. But this sort of game is a ton of fun to think about.
Art by Raffaele Giannetti, José Benlliure y Gil, and Jean-Léon Gérôme.