Note: perhaps only some of the below could be implemented. I insist that any changes be priced accordingly, adjusting up or down to make sure units are balanced both internally and with most other codices. The rule of thumb is: Eldar units should be relatively expensive, and worth every point.
Farseers: Alongside with GK libbies and tzeentch Sorcerers/daemons, these should be the premier psykers of 40k. I'd like to give them the option of having 1 more power and some powers improved. Another upgrade (that you can't get if you cast 3 powers) to make them slightly better in combat would be nice to represent master warlocks, or master bonesingers, or whatever other farseer specialization you have for seers who had spent centuries on a certain element of the seer path.
- warlocks: taken and allocated as wolf guard. Make warlocks a bit more expensive and better in melee. Have the option for other seers - i.e. bonesingers and some other types like mini-farseers. Consider allowing them to join more squads, i.e. aspects. Make better psykers to reinforce the eldar's psychic superiority.
Autarchs: More special skills you can get AND a choice of better wargear. Just because they are commanders and leaders doesn't mean they can't get something better than a power weapon - ask IG officer, Tyrannid hive tyrants, GK masters and so on. If I want to make my autarch both a better leader AND able to pull his weight in a fight, and I get to pay, say, 160 points for it, why not?
Avatar: EW. Options to make aspects better in combat - it's their GOD, people. Lysander makes marines fight better because he's famous - that doesn't nearly compare to the kill-orgasm the avatar should give aspects. FC and FNP (the latter maybe only for exarchs) as a bubble could be nice - of course, we should pay for it. Something like the BA priests would be nice - after all, this is a 0-1 HQ unit so it's not like we can spam it. An upgrade turning the Wailing Doom into a low-AP flamer (eldritch/demonic flame > power armor, I'd say) would also be good.
Phoenix lords - 5++, 3++ for asurmen, maybe a minor cost adjustment. I'd like to see them make aspects of that kind scoring, if not troops (so you can have specialized lists)
Wraithseer: please include. Wraithwall armies can be fun. Also, they are expensive, which is good for GW.
More HQs: Harlequin avatars. Think harlequin meeting DE succubus. Murder on speed.
Wraithguards: slightly longer range on D-cannon, melee option, T5 W2. Cross between paladins and grotesques. Expensive. Include an option for FnP (it's freaking golems, people) - either via upgrade or via the wraithseer
Banshees: something that makes them kill things better (i.e. reroll misses on charge, FC, etc) . Priced accordingly, of course. Banshees are narrow specialists, they shouldn't need support to be darn good at the only thing they do well! Rule of thumb: in a close combat, banshees should beat any marines of equal points - S/T4 and 2/3+ means the marines are, if nothing else, a lot more resilient and thus less specialized.
Scorpions: see banshees. While they are imo better at banshees at their respective schtick, they aren't quite good enough - being specialists, after all. Consider giving them back haywires as optional, what with them being ambushers and showing up in the backfield, and/or making them fleet like incubi, some marines, etc To focus on the ambush specialty, have an aspect upgrade skill to make them better if they attack a unit that's been pinned or already in combat (i.e. scorps can reroll to-hit rolls)
Harlequins: adjust price slightly due to rending not being as good as before. 18-20 points with kisses, perhaps?. More wargear would be nice, as well as mimes being special infiltration meisters. Consider giving a WWP (harlequins are renowned as being skilled in using the webway, perhaps even more than DE)
Fire Dragons - maybe increase cost slightly. Give them the option to trade all guns for flamers: the theme is annihilation with fire from up close, no one says it needs to be only tanks
Avengers: better options to tarpit or torrent. Price accordingly. Give them plasma so they can actually do melee well (part of their schtick is defeating the enemy hand-to-hand if need be) and consider giving them defensive grenades or haywires as optional upgrades. Exarch option with shuriken cannons would be nice.
Guardians: 18 inches catapults, with Assault 3 lasblasters as upgrade. 6-12 per unit, can be changed into storm guardians (pistol+ccw, grenade). Option for up to 1 heavy platform, 1 support platform for every 6 or 1 special weapon for every 4 (introduce another SW option). Price accordinly -shouldn't be better or pricier than DE warriors. OPTION to upgrade into black guardians (WS or BS 4, Ld 9) or give 4++ armor for a price and with limits.
Jetbikes - make slightly cheaper. Pistol + CCW would be nice. Option to upgrade to "veterans" (wild riders => black guardians on jetbikes, essentially), again with limits.
Rangers - make slightly cheaper. Pathfinders get to reduce cover a bit, maybe have the option for a WWP (again, they are supposed to be particularly good at navigating it).
Vypers: multiwound jetbikes if hornets are in, otherwise make a bit cheaper and better.
Hornets: please include. Not-too-light-skimmer with high speed and firepower: that what eldar vehicles are all about.**** you IG with their cheap, fast, uber-gunned gunships, fast and hard-hitting skimmers that aren't 10/10/10 is our turf, so **** off. We don't get 50-model blobs, do we?
Spiders - options for templates (not cheap) or CCW loadouts. Exarch skill to make DS or jumps more reliable. Surprise attack to let them charge after DS or make their shooting more effective just after they come in? Rending would be nice. Yes, they shouldn't be cheap.
Hawks: Assault 3 lasblasters, pinning, can reduce cover saves. Make them a bit cheaper. They are the aspect that does some damage, but also weakens the enemy from afar - by pinning them, haywire grenade launchers, or just some weird rules.
Spears: a bit cheaper, a bit better. 35 points for T4 3+ is a lot. Allow them to take more models, i.e. up to 7 (guys, seriously, terminators can be up to 10, and those aren't exactly common either).
Most vehicles: either BS 4 stock or a decent-priced upgrade (i.e. 10 points). Brightlance either becomes 20-25 points as is or 25-30 with AP 1 (high tech => effective at blowing up tanks) so it stops being an almost universally worse lascannon. Starcannon becomes a bit cheaper (20-ish points) if S7 AP 2 or heavy 3. EML gets a bit cheaper too. Holo-field either means the enemy rerolls to-hits, that the vehicle always gets a cover save or the enemy BS 1 is treated as if 1 lower (the latter 2 should not cost 35 points). Vehicle defensive guns should NOT be 12 inches - so either improve catapults or give eldar vehicles something else.
Prism: focused shot loses blast, gains lance. Yes, it means 2 prisms focused should be insanely good at blowing up a single tank - considering that you need to buy 2 prisms and have them both shoot, what do you expect? That's a single shot and 2 prisms cost as much as a land raider, for crying out loud. For a single shot, S9 ap 2 lance isn't the end of the world. That's what the DE voidlance is: they have a craft that comes with 2 such ones, stock, and DE players tend to still choose the ravager over it. Hint: they have a reason to.
Serpent: becomes slightly cheaper, the energy field becomes an optional upgrade. A way to make the unit inside assault if the transport has moved (how exactly are marines better at mobile warfare than eldar?); whether by giving the aspect the rules or by an upgrade to the serpent. Yes, the eldar should have an assault transport. If BA could get a fast heavy skimmer, we can borrow something from them too.
Firestorm: I don't mind it being expensive, but make it good. This thing should be nasty against any skimmers or fliers.
Warp hunters: shorter-ranged than the prism, with a very powerful cannon; essentially the eldar version of the Vindicator. Yes, this should be in.
Wraithlords: cheaper guns, especially on TL ones: 40 points for a Bright Lance is ********, 80 points for a single TL one is beyond a joke. Option for better CC, i.e. if you take 2 swords (second gives +1 attack, +1 i) or a shield.
Reapers: slightly cheaper, slightly better guns. 35 points is a LOT for BS 4 T3. BA Devastators are 26 points for a marine with a missile launcher, and are still cheaper than reapers if they have a plasma cannon. Have the option for the unit to be bigger - i.e. up to 7 models.
Heavy weapon platforms - make the spinner large blast, the vibrocannon +1 S and the D-cannon +6 inches. Price accordingly and give them the option of having a larger guardian team of gunners or allow a single on in a guardian squad if it doesn't take either a support or a special weapon.