Negotiation System

Recently I was listening to MCDM's Q&A sessions about their upcoming RPG. The designers have now mentioned something they call the negotiation system multiple times, with patience and interest as axis along with an NPC can oscillate. I ran across a moment in one of my games during which a player attempted to persuade a young green dragon to work for the party, and the situation didn't progress as smoothly as I wanted. The idea of negotiations providing a specific gameplay structure or system struck me as something I could use right now based on that and previous NPC interactions with that group. So I went ahead and developed my own negotiation system, inspired by the concepts mentioned by the MCDM designers. I have no current access to any playtest or design documents from MCDM, so I have no idea how their current or final design will function, but here is my own independently developed system that should be adaptable regardless of your current game system (but it is geared towards 5e because that's what I currently play). You might have a harder time in non-skill system based RPGs, but you are the best judge of what your particular RPG's persuasion or social interaction check-type actions are that can influence this.

The Negotiation System; or "Behold my OG skills developed through long computer lab sessions with Microsoft Paintbrush as a child"

An NPC with whom you can negotiate has an Interest and a Patience score. The starting score in a given interaction is determined by the GM. The NPC must have an interest of at least one for them to interact at all with the player characters. Whether or not they do not have an interest of at least one then determine the NPCs starting Attitude of Hostile, Neutral, or Friendly to determine how they interact (i.e. attack, or the tone of how they ignore the characters' overtures). You can roll randomly on a d6 (1-2 Hostile, 3-4 Neutral, 5-6 Friendly) if you want, but you'll generally have an idea of the NPC's attitude in advance. Through roleplay, skill checks, or a combination of both (or however your RPG of choice functions) the player characters can increase an NPC's Interest, and potentially lower the DC of a skill check to persuade them to take a desired action (again, this is all set by you. Some DC should still be 30 and decreased only to 25 if the decision is not optimal for the NPC. Your bard could persuade them, but it should be earned).

As the players talk or make checks, the NPC's patience degrades by one to two points (depending on how the interaction relates to the NPC's Attitude and Desires) per interaction or check made by the characters. If it decreases to zero, the NPC is no longer interested in listening to the characters. You can also decrease it if you feel the conversation is dragging on. Talking about pitfalls can also decrease an NPC's Patience and Interest, potentially provoking the NPC to attack the characters or at least end the interaction. Tailoring roleplay to an NPC's desires can increase their Interest and maintain their Patience levels.

Terms Defined:

Attitude: Feeling NPC has towards the player characters. Ranges from Hostile, Neutral, or Friendly.

Pitfalls: Some topics are non-starters for an NPC. Others are sensitive and likely to decrease an NPC's Interest or Patience significantly (by three or more points). Players can conduct research through roleplay or actions like domain checks (if you use domain-level play like in Kingdoms & Warfare).

Desires: Topics that NPC is very interest in, and desires information or assistance to achieve. Discussing these topics can maintain an NPC's Patience level for a time and potentially increase their Interest level.

Interest: The number assigned to represent how much an NPC desires to speak to a character. Can move up or down depending on the actions of the player characters. Once the interest reaches zero, the NPC is no longer interest in conducting Negotiations at all.

Patience: The number assigned to represent how will an NPC is to continue speaking to a player. This number can move up or down depending on the actions of the player characters. Once the Patience reaches zero, the NPC no longer has the patience continue negotiating with the player characters.

Further Customization: 

The Interest and Patience scores do not need to be on a ten point scale. Depending on the situation or the personality (or even stats) of the NPC you can scale these scales up or down (as an example, the Ents at Entmoot had much higher patience than your average crime boss).

You can easily keep track of an NPC's scores and personality by writing a description detailing their motives (i.e. Desires, Pitfalls, Interest, Patience) stored either digitally or on an index card and use two dice of appropriate sizes as your respective Interest and Patience trackers.

This system, like any system supporting roleplaying is subjective. It does not proscribe what a GM should do in any given situation. They need to understand the motivations of the NPC and attempt to communicate them to their players (for instance, I thought I made it pretty clear that the aforementioned green dragon wouldn't be forthright in negotiating when I had it bite a dragon cultist's head off as they knelt in obeisance to the monster--but apparently that wasn't clear enough). It does however give you a mechanical framework that you can communicate to your players (I.e. you should let them know they are in a negotiation and the basic framework). You can hide what's going on "under the hood" as it were unless they make some sort of Insight check successfully to gauge the NPCs reactions (look a mechanical reward for an Insight check! You can actually give them the number and say "they're getting tired of talking to you"). But this way there is a definite cut off to the negotiation so the roleplaying doesn't get drawn out and stale as your other players start to play tic-tac-toe (yes, this happened during the conversation with the green dragon. I make terrible mistakes as a GM all the time).

Hopefully this system helps you in some way. Let me know if anything doesn't make sense or needs more detail in the comments section.