In an investigative an intrigue based game, particularly a PbP one failed knowledge checks can really screw up the plot and drag things out. Knowledge skills are removed. Instead you pick 3+2xYour INT modifier 'areas of expertise.' (for example: Military tactics, Sigilian Law, Magical Theory, etc. You know a great deal about these things. Anything you don't get through this exposition you'll have to research. This could cost money or it could be a quest of its own.
Certain skills like diplomacy and bluff will be largely governed by circumstance bonuses. Skill rolls will help, but they are not the deciding factor.
I will discuss this in more detail later. Basically, movement is simplified into a flowchart so a grid-style map isn’t needed (there will still be a map of sorts, but it’ll be more general than a 5 foot grid) and you the PCs have considerable leeway in affecting your environment and pulling crazy stunts etc. This is loosely based on the combat system seen in some FATE 3e products.
Encounters with a CR aproximately equal to the party's level (give or take) can be narrated with no dice rolls but require a back and forth between you and me. At the end you will receive some consequences depending on the CR (HP loss, etc.). Anything a lot lower can be narrated by the players with no consequences. Don't go overboard. Anything around party level +2 to +8 (you should be running in an encounter level that is party level +8. Just so we are quite clear.) will be rolled. Anything larger than that will be narrated by me.
Recharge magic (from Unearthed Arcana) is in effect. Certain spells’ recharge times may be increased, and certain spells may be given recharge times. This is normally a huge boost to casters, but since the party will almost always only be dealing with 1 (maybe 2) encounters per day it actually prevents casters from going nova and also stops people from bypassing my obstacles with spells like teleport constantly.
Power keys and various planar effects from 2e will be used, though the game is centered on Sigil so it won’t come up often. I will tell you about power keys/spell keys as it comes up in-game.
GP is a metamechanic, and magic items can be fairly easily refluffed. This means that a sword you 'bought' may just be special (and completely unmagical) or well made. You can't get expensive magic items at a shop anyway. They must be made or bargained for (etc.). This also means that your character's WBL may not correspond to their mundane wealth (houses, servants, fancy clothes, etc.)
Item creation is exactly the same as buying an item (since your GP isn’t real wealth anyway). Paying for power on credit is stupid, particularly in a slow game format like PBP. Anyone can take a feat called “Item Creation” This allows you to make any item you could reasonably afford. There is no spellcasting requirement, and spells aren’t required in the process. You cannot do this in the field—you need the right tools and facilities, but you can make magic items in 8 hours per 5k GP provided you have the feat and the tools (and the money).
Disposable items are handled slightly differently. You can designate an ‘allowance’ for disposable items. Whenever there is a long break in the story you can switch or refill items as you choose. In the long run this makes cheap utility items very common and less draining on your resources. You may spend 1000 GP to add 250 GP to your allowance. For example, if you spent just that 1000 GP you could have 4 1st level scrolls. If you used three before a major break you could pick up 3 new scrolls at no cost provided you never exceed your limit. Please don’t abuse this rule.
Belief Points (BP):
BP are a short-term reward and a tool for a character to influence the game in ways outside those your class would suggest. Before you get any BP you must first define what your core beliefs are (this is Planescape, belief is critical to everyone). This is fairly simple, and you should write a few sentences or even bullet points. Even if you belong to a faction, your beliefs aren't 100% in line with everyone else, and you should explain them instead of just saying x faction's official line. You get a little bonus for summarizing your beliefs with a quick sentence or quote (from a real source or your own mind). Characters will be awarded extra BP for beliefs that are interesting, well phrased, or well-illustrated by a quote. You can also get BP for reliable and good posting or other instances of good RPing or writing. On the other hand they will be taken away in the event of unreliable or excessively short posting. Long term rewards (mechanical bonuses and other goodies) are also included in this game, but you will actually level up whenever it suits the story.
For example: Simon the Godsman believes that he is capable of overcoming all the obstacles in his path with the fire inside his heart and his drill (a few more sentences or bullets might be good. I don't have time). I think everyone knows the quote he'd choose. Alternatively, Dustbin the rogue Modron (FoO) believes that all actions and perceptions in the multiverse are the result of the transfer of information through a series of pathways and that one can manipulate reality by altering the information passing through them or by redirecting information somehow. "It's a series of tubes..."
Application: Characters have a maximum number of BP equal to their character level +2. BP can can be spent to make things happen. Costs change depending on how significant any given action is and how much narrative control it gives the PC. That means using the same ability different times may result in it having different costs each time. You may divide your pool of BP into two groups. Your personal BP are (almost always) untouchable by me. You also have a second pool: BP I can spend. Why would you give the GM control over some of your BP? Because it's considerably cheaper. Abilities that I can plan for in advance and control the use of. You can also 'decline' many of my uses to get 3/4, all or all-1 of your BP back. The return depends on how much of an advantage knowing you could use your beliefs provides (divination is especially likely to get a partial return--you benefit even if you follow a dangerous course of action). Certain rare faction abilities like the Dead Truce are constant. These will always cost BP when they come up (they are also very, very cheap) even if it comes from your personal BP. If they don't provide any benefit or if you somehow make them useless you get your points back.
Let's look at a theoretical example again: Factol Rhys' GM spends 8 points of her second pool of BP and suggests that it would be a great time to leave Sigil on a vacation to her faction's headquarters in Elysium. This is a highly significant warning (since ignoring it will probably result in her mazing or death). Rhys' player can choose to decline to receive 6 points back, and must stay in Sigil (to a cipher this represents ignoring your instincts) though you are still warned that something big is going to go down. Alternatively, Rhys' player could accept, pay the eight points and leave. Other things included in this: A signer's ability to make reasonable (or unreasonable for a higher cost) alterations to reality, a guvner's encyclopedic knowledge or reality warping, a Baatezu's ability to find a loophole, or to make retroactive changes to one's actions and preparations to represent a 'Loth's ability to play Xanatos roulette (an extremely expensive ability) etc.
A general idea of BP costs (In general a GM-provided use of BP costs around 3/4s less, but this may vary):
1-2: A minor change that doesn't have a lot of significance or a mechanical bonus to something on the order of an action point (see UA)
3-6: A plot-significant action but not one that fundamentally changes the game. Duplicating the effects of a level-appropriate spell even if you can't cast spells is possible provided it fits with your beliefs.
7-10: A plot altering action--one that forces me to change my notes in a big way. A large mechanical bonus or the ability to take a large amount of control of the plot.
10+: Like the above only bigger. It is probably unwise to burn most or all of your BP in one fell-swoop. You may want them later and regret doing something like this.
ECL for various powerful races is determined with a modified (and somewhat more conservative) version of Frank and K's approach. In general this makes monsters more playable though in some cases further tweaks may be nescesary to make your character work. In general, you can expect your ECL to be somewhere between CR+0 and CR+3. Creatures with many HD will have them reduced to match their CR, and will receive untyped bonus HP. It is possible to reduce your effective level by up to 2 over and above this (on a case-by-case basis, and you can’t reduce it past a +0) by reducing your character's point buy.
The 3.5 SRD online including Unearthed Arcana:
http://dndsrd.net/unearthedArcana.html
Planewalker.com (You can find the PSCS material for 3.5 here):
http://www.planewalker.com/