There are many different ways to approach the business of writing (and rewriting) rules, so I just wanted to throw a little out on the subject.
Firstly, and most importantly, the K.I.S.S principle. I'm sure you all know what the acronym stands for - Keep It Short and Sweet [Or Secret and Safe, yes Gandalf...]
For me, this is the overriding principle of writing rules - if the rules aren't clear and easy to follow, then who's going to bother reading all the way to the end?
This doesn't preclude more exotic answers where called for, such as where the rules don't have an existing precedent that allows me to do what I'd like to do...but even in this eventuality, in the case of Eldar, it's best to try and look at older Editions of the game for answers, before making things up.
Failing that, the next recourse is other 40k armies, and of course those written by the same guys - the High Elves, most especially.
One of the most important aspects [no pun intended] to this for me is BRB pg 74-6. That's right, the Universal Special Rules. (Seriously, those of you that knew that without looking, much respect!)
Clue's in the name, really - these should be the default solution to the majority of army-specific rules that I need to convey, I should be attempting to minimise the additions made to the game, and use pre-existing rules instead.
Naturally, I know full well that even if we managed to create a perfectly balanced Eldar Codex, the number of people who'd be interested would only be slightly larger than the number who'd agree with it, and that'd be a little larger than the number who'd ever even contemplate using said rules.
You could be forgiven, then, for accusing us here at MWFTW of mental masturbation, of simply indulging ourselves fruitlessly...and to an extent, that's true. Certainly, if you're writing rules you know aren't likely to be used by anyone outside your circle of friends, then you must be doing it for yourself, and that makes it look like an ego trip.
Thing is, with the Internet being what it is, there is almost certainly SOMEONE out there who will Google (other Search Engines are available, but let's be honest, no-one uses them) Eldar Codex Re-Write someday, and find our offerings...and maybe they'll use them.
Yes, sure, GW might re-do them before that...but then, they might go under too. Economy's pretty rough lately, you may have noticed. Uncertainty made worse by the insanely partisan governments either side of the Atlantic, and some intense fiscal irresponsibility beforehand, means that purveyors of luxury goods like G-Dub are hardly in the best position. Unlike an incredibly vocal minority, I don't hate GW, and I don't want to see them go under. I'm a realist - I know full well that if they do, interest in the game will wane, and even if I keep playing, I won't have anyone to play against. Then I won't even be able to sell my models, because I could never accept the pittances offered by the dozen-or-so who'd still be playing in 18 months time.
...I'm digressing of course. The point is, even if this Codex served no purpose other than to give me a reason to dust off my Eldar, and to paint/build some of the units I have lying around (I estimate I have about 2.5k of Eldar literally lying around between bits boxes, unopened blister packs, scattered, half-painted, among carrier cases etc. Maybe more.) Well, that'd be a Win, in my book. If someone somewhere sees it, and takes some of the ideas, and concepts, and improves their own gaming experience as a result, even better. Even promoting discussion about how exactly people SEE the Eldar as a race, and how they should function on the tabletop, is interesting to me. I'm a cerebral person, I have an oft-crippling tendency to overthink things, so thinking about things like this helps, apart from feeling entirely natural.
So - simple answers, even to complex questions, where possible.
To an extent, this extends to a rather minimalist design. Trust me, I could create new Aspect warriors all day, new weapons, new units, new vehicle upgrades, etc etc. Simply put though, I don't see the point. The Eldar Codex currently has 27 units, counting Phoenix Lords as a single entity. Codex: Blood Angels has 41, if we exclude Corbulo and Lemartes. So? It doesn't make it within my purview to start making a bunch of things from scratch to "even the score". One or two things, sure - but ideally things that already exist within the model range, such as the Nightspinner, the Firestorm, the Warp Hunter...I see my function as that of a surgeon. I'm here to stop the internal bleeding going on - but it's not my job to graft on extra limbs, or add extra organs, just because they'd be useful.
If something cannot be reduced to a simpler version, then it comes down to the real nitty gritty...is it REALLY necessary? If I can imagine the unit or army functioning the way it does in the fluff (within reason, of course, balance cannot be ignored at any stage!) without the rule, then simply cut it out. If that isn't plausible, or if the overlap threatens to become overwhelming [for instance, the number of Eldar units that could quite seriously benefit from Hit And Run is silly...] then a new approach needs to be taken. Perhaps this is a multi-tier approach, such as Power From Pain or Mastery Level, or perhaps it's a more composite approach, such as [for lack of a better example of the top of my head] Descent of Angels, which is actually two rules built into one.
This will naturally lend itself to a somewhat minimalist design in terms of options too. I'm all in favour of what you guys said in the comments, and the Aspect Squads being 'Base Cost', 'Unit: 4 x and 1 x Exarch' rather than making it a mandatory additional upgrade. Width preferred that layout because that was how GW used to do Codexes, but I'm perfectly happy to make Exarchs mandatory, because it's simpler. Does it restrict player choice? Well, I suppose it'd be an entire other topic to go over - so we'll not get into that right now.
Another example though, is in the in-built Defensive Grenades on Guardian Defenders and Dire Avengers. They need them. Simple as that. There's going to be very few instances of players NOT purchasing the upgrade [assuming this Codex were to supplant the other, in this thought process] so why bother to not build them in? Especially since Guardians are difficult to point appropriately, in relation to other Codexes. They can't be as cheap as an Ork Boy, that's silly. Yet, Guardsmen are cheaper, and Guardians are essentially the same...just as survivable, anyway. Eldar have an issue though, being that they aren't numerous. Certainly, GW has gone a long way recently to expand the total number of remaining Eldar so it possibly even reaches billions, not the hundreds of thousands once heavily implied by the fluff - but the point of Eldar is that they're an Elite force, like Space Marines, and will be outnumbered, unless you're literally attacking a Craftworld. Having spammably cheap Guardians runs counter to this principle of what makes ELdar who they are, and so the alternatives are two [as I see it] :
1) Make them better, to justify the cost
2) Give them more/better gear, but leave the stats alone.
The third option:
3) Leave them crap...
- That simply wasn't an option. Eldar are a combined arms army, they should often, but NOT always, include Guardians to bolster their numbers, while we still make a Swordwind army easy to field.
Making Guardians better wasn't a true possibility either. These are civilians - I'm not sure exactly how they compartmentalise the horrors of war, and of murdering other life, even in self-defence, but they find some way to do it, without the Aspects' War Mask, and without the intense physical training. Equipment it was then - the simplest solution.
Okay, this will be a long enough read as is I reckon. To be continued, I suppose...