Perhaps a good thing to consider throughout the creation process is how can the players fail? And once they have failed, how should this effect the players?
In most systems, failing an attack roll does not kill you. Failing casting a spell does not normally kill you. It's the opportunity granted to the opponent by allowing them to continue acting for a turn that brings you closer to death. However an unbalanced system from my experience is a social system.
Unbalanced? How so? My players gather information, persuade folks to buy their stuff and lie to get out of trouble. The see through my every attempt to lie to them, use their superior status to intimidate them and gather minions. However, it isn't about what the players do, it's about what the NPCs do. When a player bluffs, the NPCs get their normal defense initiated by the PCs offense. Occasionally or all the time depending on the DM the NPCs lie and the players must actively see through the lies with statements like, "I check to see if he's lying."
Compared to combat it is a completely different game. More like a shooting gallery than a battleground. Failure results in not getting information, then the game is over. Odds are there are no losses during a social check. Of coarse, now it should be the opponents turn to strike. Suddenly the PCs are subject to a diplomacy check for once. What will they say when they're told that while they were trying to sell their loot for more, not only did they fail but they've just been persuaded to buy a 'magic carpet.' Forays into the market just got a bit more interesting eh? Risking their gold while engaging in haggling is one option.
What about the PC getting cornered while wandering around using gather information in the big city? Now they're being intimidated into giving up their valuables. What happens when the intimidation check succeeds? How would the PC react to being forced to give up their precious family heirloom sword?
In my personal system, this would be unlikely due to the nature of the PCs being Heros/Leaders. Getting cornered and being alone when you can theoretically take on 4 normal people handily makes it unlikely to become intimidated. Your persuasive skills on the average merchant would be more than likely better. And being alone is unlikely when by level 20 you probably have an entourage of at least 4 people.
In this case it would become a social battle, primarily based on morale you would take your social maneuvers and hopefully come out ahead. The great question is, in physical combat you die. In social combat what happens? You become irrevocably traumatized? Perhaps it would be dependent on the ability?
If disintegrate kills you you are irrevocably gone. If a fireball kills you you may be a pile of ashes and bones. If you are crushed, absorbed, impaled, sliced, chopped, exploded or dissolved aesthetically it is different, and in most cases functionally the same. You cease to retain control of the character, the character ceases to function and your part of the story ends. Any plot lines exclusive or focusing on them are likely over and questions will go unanswered, quests unfulfilled, rewards left unclaimed.
An examination on social combat would suggest that having aesthetic differences and rare functional differences would be the way to go. Aesthetically they are seduced, break down and cower in fear, become completely agreeable or just plain become a jibbering mess. Functionally, the character loses control of their character until morale improves. They genrally become agreeable to the opponents suggestions, are willing to give up anything of theirs and can lose most things if not everything. Without friends to save them, they are irrevocably lost.
This may seem harsh, but so is dying. When a player becomes close to death they have an option to retreat. When their morale gets low, they will likely move to retreat from social combat as well. Surrender is also an option, and the most likely. If imprisoned they are alive and not dead with health lost, and if they retreat from social combat then they lose only what the opponent wanted, not everything at once.
Of coarse here we have dealt with social and physical combat and checks, but there are other checks, unopposed checks. Traps make checks against players, and players can make checks against traps. What about other challenges? Could working a forge with exotic materials be worthy of having the required heat fight the player physically? Could the sheer challenge of the necessary attention to detail fight them mentally? What is the failure for industry? For travel? For knowledge?
Believe it or not many of these have built in failures. Industry usually risks loss of wealth. Travel could result in falling from a cliff, increased encounter frequency, and any number of hazards. Knowledge however is more unique, should there be a price for failure? To simply say one does not know could be easy, but what about remembering false knowledge? Should this be a critical failure? Or should I add gradients to success and failure, in a similar fashion to Savage World's raise system?
I was hoping to do things a little more statistic oriented and cover some of the things I learned from putting my work together in an organized fashion. Obviously this post is not it. Work continues to side track me. Therefore it is likely that at some point there will be more to come..