I reject codified custom classes in D&D, even for classic D&D (looking at you B/X Blackrazor). I just think that this mimics modern D&D did with all of its silly and limited skill, feat and class lists. I don't want a book to tell me who or what I should play, or what the right way is to play it. I want my imagination fueling my character. Having said that, I love players that think creatively when designing a class.
One of the biggest misconceptions of Basic Dungeons & Dragons concerns its class system. Most, including myself, will tell you that this was the biggest barrier to playing. The idea that you could only play a fighter, magic-user, cleric, or thief seemed incredibly limited. Race as class was an entirely different problem which I won't go over right now, but suffice it to say, it was hard to convince me that this game was anything but a seriously toned-down edition to the "real" D&D in modern iterations that gave you more options.
Obviously I now know better. I know now that "classes" in old-school D&D are really just archetypes or simple templates to keep your imagination in check. When making a character in Basic or 0E D&D, I always suggest to come up with a class that fits into one of the 4 archetypes listed at least as a basis to start from. Sometimes classes can blur the line a bit. For example, a "knight" class could be either a fighter or cleric depending on if you wanted a character that held to strong moral codes or not. A "swashbuckler" or "pirate" could be classify as a fighter or thief depending on whether you envisioned the character focused on fighting or cunning more often. Classic Conan in literature is probably more of a thief than a fighter, which I've gone over in a previous entry.
Thinking about classes in this way, you quickly realize that that classic D&D had far more flexibility than its modern cousins. Freed from stat lists and restrictive prestige classes, you can literally come up with anything you want to play as long as the DM allows it.
One of my players loves playing a Demon Hunter in Diablo 3. Now, good luck finding a B/X book with a Demon Hunter class, much less Diablo 3's version of it (not that I'd want that anyway). So without someone sitting down for hours statting out the perfect class to fit the game, how do I achieve this?
First of all, throw out the idea that a character's class needs to be completely defined and planned day 1. If a player wants to sit down with the basic Cleric template and let the game define and mold who his character is, great! There's nothing wrong with that at all. How many times have you said, "that class looks awesome!", then a few hours in you decide you want to be something different?
Alt-itis can be completely avoided in classic D&D because you don't have to put anything in stone during character creation. It may be during session 22 at level 3 that he gets a series of lucky rolls against some troglodytes and his character suddenly "remembers" that his sister was killed by a troglodyte when he was 11 and he has a racial bonus against them. Or perhaps the DM simply decides that this character has gained some insight into killing cave-dwelling monsters and will now have more success against them in the future.
Secondly, if someone does want to put some effort into defining his character early on, he can put as much effort into it as he wants. Don't worry about finding a perfect balance. Pick an archetype that fits most closely the vision of your dream class and then add 2 or 3 tweaks in the positive and 2 or 3 tweaks in the negative. For example, I designed the Demon Hunter class mentioned above to use any weapon, but shuns armor heavier than leather and is penalized for wearing it. He can turn evil planar creatures starting at level 1 along with undead, but his undead turning is weaker and fails more often than the standard cleric. Also, he prays to and gains power from a god like a normal cleric cannot heal wounds, only cause them.
That's a great start at level 1, and even if he wants more specializations in other things like a particular weapon like a crossbow, I will allow him to work toward it by using such a weapon or skill during his sessions of play. A player shouldn't get everything he wants at 1st level, he should set goals for how he wants his character to turn out. Goals are not always reached when he hits a certain level either, it may happen during an interesting session where he did something impressive, or arbitrarily after putting a little more effort into character back-story.
Negative things can and should happen as well based on poor choices, or simply because the DM and player agree on something negative balancing out something positive that occurs. This is a collaborative effort and neither the player or the referee should be able to hold all the cards when it comes to defining player class and character.