War Walkers are the other half which make up the Hornet (the former being the Vyper) but unlike the Vyper, there is no simple fix to differentiate the two (Vypers could become light transports). Their roles are effectively the same - cheap heavy fire support but the Hornets is more mobile, more accurate and more durable for only a moderate increase in points. War Walkers do take up a different Force Organisation slot so it's less of an issue but with other good options already in Heavy Support (and more to hopefully come), they'd likely only see use with a specific build (i.e. Hornets + War Walkers). The trick here will be to make them stand-out in some way.
The Review -
So, the review! War Walkers are the Scout Sentinels of the Eldar world but with two heavy weapon points and are classified as closed environments (ergo more durable). Still AV10 though so not hard to break but a very cheap chassis with the ability to scout for some not too expensive firepower. War Walkers are also walkers, shockingly, which gives them a bevvy of additional uses standard vehicles do not have access to - the most obvious of which is to tie units up in combat (though they can also be tied up in combat) and the ability to move up an down ruins.
This is obviously quite useful when Outflanking or Scouting. One's options when Outflanking with a unit which can shoot and engage in combat are obviously increased though the issue of coming in on the right side comes back into play with attempting to engage backfield units in combat (shooting with 36" guns has less of an issue). The ability to move up and down ruins with these moves to provide better firing lines or protection from combat is also a nice bonus though remember, to get down and start moving laterally again you have to take time climbing down.
Otherwise, Scouting is less useful for War Walkers than many units - they don't have Infiltrate so cannot deploy in a super advantageous position and then Scout to safety/even better position and don't have the speed to suddenly appear out of no where. It is useful though - any extra movement is useful and it can help clear the movement lanes for the rest of your army earlier (if on foot) or simply get the War Walkers to a position they simply couldn't deploy in. Beyond that, well there's not much to say about War Walkers. They are relatively cheap and relatively fragile but offer Eldar some decent ranged support with the double heavy weapon systems.
Potential Uses -
The War Walkers have some pretty cool and cheap firepower available to them. As the only platform in the legal Eldar codex capable of being brought in numbers with two fully loaded heavy weapon systems (i.e. not underslung cannons) they are an excellent source of firepower. Their main issue? Competing AV12 Heavy Support choices in the Falcon and Fire Prism who bring better durability, mobility and options to the table (guntank with transport option, blast weapon respectively).
Beyond this though, War Walkers are best utilised with Scatter Lasers or Missile Launchers though the latter is quite expensive. Scatter Lasers are really the weapon of choice for Eldar currently due to the way 5th edition works and pricing in the Eldar codex - you get a lot of shots for relatively cheap but lose out on strength. EMLs are still a bit pricey here but on a system capable of taking two of them interdependently, it gives the Eldar army actual ranged anti-vehicle punch that is not a one off Bright Lance or what have you. Six S8 shots is nothing to be sneered at and comes with the added bonus of an alternative blast option so they are useful against infantry, too. It doesn't come cheaply though and is on a pretty fragile platform.
Outflank is best used with Scatter Laser (or Shuriken Cannon for a cheaper option) based War Walkers - it allows them to maxmise the lower strength of the guns in the side-arcs of tanks which they might not be able to get to with only their 6" move. Again, they can also tie up backfield units and being AV10 walkers, most backfield units simply cannot deal with this.
Farseers are also a great tool for a War Walker squadron to have - Guide & Fortune make them much more reliably offensive and defensively if you can get them cover. It's an expensive proposition, particularly if you've gone with EMLs, but you're already taking a Farseer more often than not - here's a good use for extra powers beyond Doom.
Potential Changes -
Something needs to differentiate the War Walkers just that little bit from Hornets. Whilst obviously having good units in both the Heavy Support and Fast Attack slots other than the Hornets and War Walkers will obviously make mixing and matching them between lists an effective option, you don't want them to essentially be the same but in different slots with different fire applications (with one obviously superior in the Hornet). A little change like Fleet is going to be nice obviously to have more impact from Outflanking (particularly if the Autarch's ability morphs to allowing re-roll to outflank sides) but isn't huge. A new weapon available only to War Walkers though would be nice - something the Hornet would kill for but can't have! An autocannon (S7 starcannon?) or multi-melta (24" firepike?) equivalent are some obvious choices here though both would be better on the Hornet where side-shots/melta-range is easier to obtain.
Perhaps swapping these choices then would be best... Either way, the result is both platforms are capable of S6 spam (scatter lasers/shuriken cannons) and one platform has access to the higher strength 36"+ weaponry whilst the other one has more middling strength/range (S7 + melta combo) weaponry. This way each platform would have a more specific role augmented by their movement but are both still capable of working within a base system (S6 spam) and within their respective FoC slots.
Potential increase in combat ability as well? Not only could they tie up enemy units in the backfield but actually beat them down? Mini-Dreadnoughts or what not...
The War Walker has its strong points but like the Vyper, they are nearly all covered by the Hornet. Two weapon systems, relatively cheap, scout, squadrons, etc. but not fast, not BS4 and not AV11. They may be half the base cost but those are some seriously nice upgrades they are missing out on. In the current Eldar codex though, they are the only platform capable of taking two heavy weapons en masse (i.e. you can have nine and not break the bank) which gives them certain uses within the army. Unfortunately without paying through the nose, this is generally what the army does already - S6 spam; you just get more of it for less!
In terms of differentiating the War Walkers from the potential Hornet inclusion into the Eldar codex, different weapon systems and improved ability in combat (+ Fleet) would really set them apart beyond their Force Organisation slots. Whether this is War Walkers getting new weapons or Hornets doesn't matter - as long as there are different sets on the different platforms and you might see both competing for spots in armies rather than one winning out all the time.