I will probably set the rules to be created in line with the setting rather than being generic enough to cover broader 'sword and sorcery fantasy'. Somewhat like Dark Sun did, that is a familiar start but some divergences as far as detail specifics and mechanics. The 'core' I think shall be B/X since it is my favorite bit, and then change what all I need for this particular setting and style of play. My initial inclinations:
Magic: I will probably do away with Vanician magic although I still like the idea of spellbooks and choosing spells. As above, this will likely be specific to the setting, whatever I come up with. Mainly I want a decent variety of spells including a fair amount of non-combat spells that can be cast without effecting combat slots. Combat spells I would still like to be regulated so they are not pounded out with little thought. So what I am thinking is most non-combat spells are not stressful enough to warrant ill effects, that is removal from memory, chaos warp damage, whatever. Combat spell, and other spells cast in stressful situations however do have ill effects, probably something along the lines of removal of the spell from memory and perhaps some other insidious effect. Personally I like said 'insidious effect' since I can place in powerful spells at an extra cost - to me that is a Wizard. It is why PC should be damn fearful of crazy Wizards.
Combat: Since the game will be pbp, email or other such online method I will likely bend combat away from the standard hit or miss method. It is very unlikely any of our sessions will be live, so combat can be written with this in mind. DM description only goes so far in my opinion and is a bit overly redundant for long combats, even with time to write them ala pbp/email. I may use some sort of 'every meaningful hit is a critical' more or less. Ala WFRP but with the table extended for lesser effects. A 2hp strike ill have some effect, even if you have 40 hp. AC will probably be the buffer in those instances where light damage effects can be mitigated(isn't that the point of armor anyhow) but hp attrition still takes place . I may write a script to test it all out, after all I can run 500 mock combats and see what the end result is fairly quick, definately a tool they did not have way back when. I doubt I insert any extra rolls in combat, just a fairly large chart for combat effects based on the factors above. It should be a fun challenge since I can be clever with things like AC, the dice of weapon damage and the actual damage dealt. Also I can put an end to things like hp inflation, awkward healing that stalls the game and so forth. After all why not have hit points reset after every combat? How ever the real injuries are a different story..
Classes: I may have players be all classes and level them as they go. So essentially a Fighter 1 would still be a Fighter 1 but have a 0 level MU, Cleric and Thief. When I read Conan, Fafhrd & Greymouser, Elric, and Dune it seems clear the main characters are far from one dimentional. What I am thinking is PCs could bring 1 or more classes up as they level depending on where they would spend their xp. XP cost would be by class level, as would any other benefits of the class. The listed B/X xp cost is actually fine and would require no changes. I am still mulling it over but I see no reason why 'Paladins' and so forth can't offer some fluff available, perhaps something like this:
Fighter Cleric - Paladin
Fighter Mage - Monk
Fighter Thief - Ranger
Mage Cleric - Bishop
Mage Thief - Illusionist
Cleric Thief - Assassin
Note that Paladin and so forth are not actual classes per say and do not level on their own. They would simply denote that the PC is a fighter and cleric and may offer some additional fluff abilities. Triple classing is more dicey and I may eliminate any title or fluff powers for triple class. If a Paladin per-say gains another class, other than cleric or fighter, it would remove said fluff powers. Or at the very least this is an area Race could come into play and perhaps offer their own fluff titles; Halfling fighter/thief/mage - hello Sheriff!
What else?: I don't really care for the idea of individual skills or feats in the game. The variety they offer is outweighed by bogging the game down needlessly. A skill driven game perhaps but not D&D. I may offer something through individual classes (ala Paladin) to give a bit of variety without adding in a whole new mechanic like skills, feats, advantages/disadvantages etc. Feats offer a decent avenue to avoid simple hit-miss combat but I think I can do so with other means rather than creating huge needless feats lists. At this time I think a general combat change and some fluff mechanics in multi-classing should offer some decent options.
And so, I am thinking those are a broad overview of the changes I will look into. Things like races and whatnot will depend on the setting overview which I will talk about soon. Valley of Blue Snails will ultimately be far more setting heavy than rules heavy so I want to keep that in mind when designing the rules. Complexity wise I am thinking above B/X but below 1e AD&D, and hopefully the complexity in the right places.