I've been angsting on and off about how to allow players to customise their character classes without tipping everything out of whack. Not that it's really been much of an issue thus far; most of my players seem to be perfectly happy with one of the base classes. However, I like to have options available.
I've been using add-on sub-classes which tack on to one of the basic classes, adding abilities and increasing the experience required to rise in level thereby. I think the concept is OK, but it still straitjackets players somewhat, and there's no real consistency in terms of abilities gained to XP required.
In it he proposes a system whereby one begins with a basic human-equivalent nobody, without any particular abilities at all. This mannequin begins with an XP progression that appears ludicrously low: only 400 XP required to get to Level 2, and 75,000 to reach the giddy heights of Level 10. The catch is that, assuming it survived to get to 10th level, it would still be a pretty useless character.
What you do is, you start adding abilities to the mannequin — things like Hit Dice (from d4 to d10), maximum level, armour and weapons skills, spell-casting (and maximum spell level capability), and even weird species-specific abilities like flight or gills tough skin or whatever.
Each of these things increases the XP requirements for level increase by a certain percentage. For example, if you choose d10 hit dice, you're already at 200% of your base mannequin's XP requirements. Add the ability to use any armour and any weapon, and that's another +125%. If you want to be able to amass HD until 12th level before dropping to a standard per-level increase, that's another 40%. If you want to increase your combat skill every other level, that's another 100%.
By now, your 400 XP to reach 2nd level has turned into 400 x 465% = 1,860 XP. And basically you've recreated a bog-standard Fighter. (Note that there's more stuff to be included, such as saves, special skills and allowed magic items and so forth, I just thought I might as well stop there.)
Crabaugh was writing to players of Basic D&D, though there's an editorial note to the effect that it could be used to slightly modify AD&D classes as well. His system is not completely suited to my own Swords & Wizardry-based campaigns, but it wouldn't require too much tinkering to make it fit, and I think I'll give it a go.
What's the worst that could happen? Actually no, don't answer that. I can already imagine.