Oh I THINK DD might have the Immortals rules too, which was a completely optional expansion book for either BECMI or Cyclopedia.

Yes, immortarl rules.

I would say that DD is like "D&D Rules Cyclopedia Ultimate edition".

Is, essentially, a better version of the same game.

Yes, one of Jack Chick's best efforts.
BTW the film version of it is available here at GOG.
And they do it perfectly, straight up, allowing the ridiculoness of the tract to speak for itself.

I would add that the weapon skills and mastery add much flavour to combat.

The THAC0 and hit points progression is so much more balanced and allow for much more room for levelling up, while keeping the game managable even at later level.
The addition of skills is nice and simple, but still players can fiddle with it a bit.

36 levels are enough for the GM to decide what will be the end of the campagin.

Non-human races as class, even if outdate conpect, still has lots of game flavour, for me at least.

I've never used either weapon mastery or skills in the cyclopedia the DM hated both and he basically tried to stick as close to the BECMI/Cyclopedia original rules as much a possible.

I can agree it can add a bit of burden at the start, but the GM can re-print the tables in a much more compact way and with a bit of experience, the combat will as much as fast, but with more fun effects.

Potentially, the GM can borrow the idea and customize the tables!

I wonder if drivethrurpg.com will ever get wrath of immortals set for the Cyclopedia, so far they only have the original BECMI immortals set which would work with the Cyclopedia but the RC version is supposed to be much, much better.

They basically are a class in basic/classic. A fighter is always a human, a magic user is always a human, cleric is always a human.

Except for one difference: The three classes you mentioned might be only for humans, but (for now assuming "thief" doesn't exist as a class) that's more classes than other races get. My idea is to give humans just one class, just like the other races get only one class.

One other idea that I have, though that would increase the complexity of the game a bit: Introduce 3e style multi-classing. Once the character reaches a certain level or so, the character can choose to, instead of continuing to advance in her racial class, choose to advance as a class such as fighter or magic-user. This would, in turn, allow players to focus their characters differently, perhaps allowing an opportunity to break racial stereotypes, but the character's race would still matter.

Of course, if introducing 3e style multiclassing, there's still the issue that multiclass spellcasters tend to have rather weak spellcasting abilities, so there'd need to be some mechanic to deal with that. For example, maybe make spells less dependent on caster level, or have other classes contribute to caster level.

(One could also limit characters to one racial and one other class if one doesn't want multiclassing to get too ridiculous, and/or one could treat half-elves (and other half-breeds) as multiclass characters (though for the latter case, 1e/2e multiclassing starts to feel more appropriate.)

By the way, I have decided that I don't really like having priest and thief as classes. I find that thieves tend to either be mandatory, useless, or dependent on positioning (which slows the game down as positioning now has to be tracked). As for priests, I don't like having healing magic be the exclusive domain of religion; I would like to be able to play a healer (and a primary healer, not the weak healers that are traditional in D&D) who learns and casts spells through study rather than belief.

In the basic book yes, but if you add the gazateer little mini expansions, you get more variations of other races that have abilities of said other classes. Yeah thief exists in basic/classic. Again a human but again there are racial variations of say elves, that can do clerical things, or dwarves that can be thieves, etc.

if you want humans to pick levels from different classes as they choose, I would not bother too much balancing.
Having a bit of everything is already a good thing, and not being powerful as specialized classes is the price you pay.
Even "weak" spells are not weak in a well played game.

Balancing is important but I think is over-enphasized from the time of D&D 3.x onward.

To add a bit more, balancing issues come out when a game engine is pushed over it's limit, to do something was not tought to do in the first place: so you add rules and exceptions to ractify other special rules and exceptions.
This go on and no, but will never solve the fundalmental design gaps.

This is what D&D 3.x > have done.
Take a simple system designed to do practically nothing and start adding exceptions and special rules by the hundreds (feats, spells, classes, ...).

Anyway, I have an interesting thought that might be interesting for a silly "joke" campaign. The idea is this:

In many roleplaying systems, certain rules go unstated as the developers expect common sense to apply. For example, from what I have read, D&D 3.5 doesn't explicitly state what actions a dead character can perform; common sense says none, but the rules don't explicitly state that.

My idea is, for this campaign, whenever the rules don't explicitly state something, take the interpretation that is counter to common sense. So, for example, characters who are bleeding to death would not be able to act (as normal), but once a character dies of those bleeds, she can suddenly act again. (Note that this non-standard rule interpretation would apply to monsters as well.) At this point, you have a game with strange mechanics that defy common sense, and I think it might be interesting to maybe play a session or two like this. (Maybe this would make a good April Fools day joke?)