(didn't have the time to read the whole thread, but still...)
My ultimate wish would be that only class is mandatory, and everything else is optional: skills, feats, backgrounds, equipment, races, alignment, subclasses... all of these could be "modular" and the game still function with them (at least on a group basis, not necessarily as an individual choice), if the designers wanted to make them so.
Skills effectively expand the complexity of ability checks. I love skills and I hardly imagine playing without them, because I love to have different characters with their own narrow fields of expertise, but there is no reason a gaming group couldn't play a D&D adventure effectively without skills, in fact that's what happened for almost 20 years before non-weapon proficiencies!
Feats have always been add-on abilities, thus they are modular by nature. I do not understand why Mearls have decided to make feats mandatory now, I think it's a mistake. A gaming group may not want to use feats in order to keep the number of character's feature low, thus making the game less complex.
Backgrounds have been introduced to represent the concept of "what your PCs do when not adventuring" (or "what your PCs used to do before adventuring"). Clearly, if your gaming style is focused on adventures and ignores the time between, you may not want to use backgrounds.
When I say Equipment should be optional, I simply mean that there might be a gaming group which is uninterested in differentiating weapons. A default weapon damage (like in OD&D, IIRC) and perhaps a default armor or two would be enough for such group.
Races can be optional too, in fact we didn't use them in our playtest! Not using races doesn't mean you can't play an elf, it just means you're not going to add racial benefits (not even human's) to your character sheet. Nobody ever mentions this because it feels weird to say your characters don't have a race, but those benefits are all perfectly additional and the game works totally fine without them, if you want to lower character complexity.
Alignment is optional only as long as other stuff in the game (most notably spells) doesn't have mechanics directly referring to alignment, which hopefully is going to remain this way.
Subclasses can also be optional if done properly (granting additions/variations that are not strictly required), but right now I think they aren't like that, for example because a Rogue without subclasses is just someone with sneak attack, which hardly makes it enough of a Rogue. Most notably, subclasses would be really be optional only if they were balanced across different classes: right now, taking away a Paladin's Oath or a Monk's Tradition doesn't hurt them much, but taking away a Cleric's Domain or a Rogue's Scheme is a bigger deal, so you can't easily play without subclasses with these characters at the same game table.
It may not be a big deal for me, since anyway I'm probably going to use all of these options anyway at my table, but the game would be better for other groups if these were optional, so I wish they would ALL be such.
OTOH, I definitely want ALL of these to be available in the first 3 core books!!