Customizable classes
Well, if you're doing a conversion and not creating an entirely new game, the customizing thing brings about a little difficulty (unless you want to spend the time to break up all abilities and give them ratings). My suggestion for greater customization in D+D would be to come up with an experience/purchase system.
Characters create a character as normal. When they reach 1,000 xp, they do not automatically gain a level. Instead, xps are saved until the player decides to trade them in for stuff.
Cost...................................Benefit
xp for next level.................Gain another class level*
xp for next level/3**..........Gain either Save, class abilities, skill points, or Base Attack for next class level.
xp for next level/2**..........Gain HD roll and Skill points for next class level.
500***................................Extra feat
*Only this option may be used to determine true class and character level.
**all class benefits (abilities, base attack, saves, hd rolls, etc) that are gained without actually gaining the full level may only be applied to classes the character has at least one level in. When a class benefit is gained without raising the full level in that class, the character is considered to be a level higher in regards to the gained benefit only.
***The cost for feats doubles each time a feat is purchased (500 for 1st purchased feat, 1,000 for 2nd, 2,000 for 3rd. Feats and bonus feats are still gained for raising class and character levels as normal.
Yes it's quirky, confusing, and full questionable mechanics...but it might be a good start towards a more customizable D+D.