I noticed a lot of people have a problem with Castles & Crusades SIEGE engine. I've also noticed from playing with my players that the original d% system for Thief skills is extremely easy to use. Furthermore players love it, because they know precisely what chance they have of succeeding. There's also no need for adding modifiers on the go, and it definitely cut down on powergaming. Even my more 3E oriented players loved it.
So as an alternative to more complex systems, simply make most class related abilities a percent roll. I'll leave the details up to you, but for myself I've even eliminated the table for Thief skills. Instead all skills for any class start at 35% (25% if you want a tougher game), and increase by 5% every level till they reach 90%, at which point they increase 1% per level.
While this does sacrifice some granularity, it is extremely simple and quick. You don't even need a table. You can have ability scores modify if you like, but with my 35% system I already took that in to account. If you look at the Thief skills in AD&D, if you add in the dexterity and racial adjustments, you get around 25-35% for the average of skills.
Of course there are non-skill class abilities as well, but those are usually described within themselves; either being at-will, daily, spell-like, etc... This would simply be for skill abilities like "hide in shadows", "pick-pockets", etc. This can also be used with Adventures Dark & Deep for the skills of classes like Bard, Jester, Acrobat, and Mountebank.
I've also finished mocking up a nice house rules document for myself. It's just Swords & Wizardry Whitebox, with certain clarifications and additions from S&W Core, and Basic Fantasy. It's all technically open game content, so when I've formatted it to my liking, and mocked up a cover, I'll be sure to share. Obviously I'll be releasing a supplement with the above material.
If I do create a supplement look forward to the following classes: Bard, Acrobat, Minstrel, Jester, Mountebank, Barbarian, Cavalier, Paladin, Ranger, Witch-Hunter, Scout, Martial Artist, Assassin, Mystic, Druid, Illusionist, Savant, Alchemist, and Psychic. I know it sounds like a lot, but each class is basically a collection of 5 to 10 abilities, usually very simple and short in description. I might post soon about each class to give an idea what my partner and I have come up with.
Right now I'm looking to mock up a set of reference sheets that quickly lay out what each class can do, nothing publishable, but something to play with. I already have hand written notes on what each class, but I don't have an explanation for any of the abilities yet. Of course this is all subject to demand, of which there is very little.
