This has led me to Chess as a major design element.
(this has nothing to do with nothing, but man this thing will stick in my head!!)
Chess is simple, (more or less) balanced, and unless you got the special edition chess board, easy for a player to quickly figure out what's going on. Now, I can't quite off that level of simplicity with an RPG--nor would I want to. This is a hobby that does better when we can add components on top of core rules to make something special. Like Cooking--but that's another post...
What I can steal from chess is the idea of unit Ranks and Movement. Combined with the idea of Magic cards, and I got a whole lotta gas to cook with.
From a story point of view, Pawns are the henchmen, the front line guys that rarely get a hit in on the hero--but that's not their job. Their job is to slow the hero down and increase the sense of peril our heroes are in.
Then there are the "important" pieces of the back row. First, you have the "obviously important but not named" characters. These are the support bosses, the Imperial officer bossing around the storm troopers, or that guy in the old TMNT movies that barked things like "Ninja...Vanish!!" and vanish they did. By themselves, pawns are not that threatening physically to our heroes, but a good support unit can make them a much bigger threat.
Then there's the Special units. The one that your evil boss says "Unleash the Flying Monkeys/war-dogs/guys in red armor/tax attorneys!!" before sicking on you. These units probably have some special effects to back them up, and normally are just meaner and more deadly. Heroes will still probably win against them, but it might be at a cost.
Now, maybe you need more than Special--maybe you need a Big Bad Monster. Those things that are more special effects than characters, a veritable tidal wave of doom. Closer to forces of nature, something that nobody's going to be happy seeing.
Finally, there are the Named characters, the Champions--the guys who might actually get billing in the commercial for your game, or at least an action figure. These guys are at the level of the people they're going up against...if not the entire group, at least a singular hero.
These are broad and wide descriptors to be sure--but it's a place to start, and frankly I kind of enjoy the idea of building an army to stop your four PC's. Ranks help--they give the GM and the player's a glance of what they're dealing with, and how serious the situation is.
So now, back to Magic. As PbP and simple are two goals for me, I've quickly fallen to "why am I rolling to see if Thug #194 really hits you?" No, instead we're going to make sure that you can fit your villains into a simple spread sheet at least for a combat round. Quite simply, we don't care what the Skeletal Archer's social skills are like, since that's not why we brought him here.
Instead, the goal is that every NPC that you might fight will basically have a combat bar--This is their Melee, This is any ranged they have, this is how many hits they can take--basically a magic card description. Like the Magic Cards, we'll have room for special attacks or random things like "flying" or "Really scary" or "uncanny knowledge of the Dewey Decimal System", etc. Basically, we're looking for enough wiggle room for unit's to be unique to each other, but not so complicated that we all need special rules for every action other than "stab".
In general, the real trick of it is this--games should be simple enough to pick up fast, but complicated enough that you need to think about what you're doing to really get the most out of it. I know this is close to "salt is salty" when stated, but I think it's an easy thing to forget about.
I'm trying to build up an actual example of what these things might look like in case any of you are interested, so gimme a few days. This is the last crazy week for a while (until the next thing that makes me have a big crazy life), so I'm hoping to be able to really plug at a few things shortly.