Recently Relic has proposed doubling the slow of Devastator Heavy Bolters. I'd like to outline why this proposition would accentuate the balancing issues this game has betweens faction right now, and an idea that might help alleviate it.
Heavy Bolter teams have a slow straight off the bat, with no conditions attached. They also have the highest single target DPS of all the HWTs, at 56 DPS.
Lootas have a slow, but only after looting scrap, a condition that means that having this special ability on one unit means, in the early game when scrap is less available, that you are trading a special ability on one unit for slow on Lootas.
Dark Reapers, meanwhile, have no slow available to them. They also have the lowest DPS against individual targets, at about 33 DPS. While they have True Damage, there is surely no argument that True Damage compensates for lack of a slow, since they also suffer from significantly lower DPS.
Relic also proposed changing the Reaper Focus doctrine to provide a slow under Fleet of Foot. I have a huge problem with giving a utility to Eldar in the form of a doctrine when BOTH other races get them without requiring one.
When I look at the other Eldar doctrines, I see a lot of abilities that are, for me, pretty much required for me to use the unit effectively. If I don't take Improved Scream, I won't build Banshees; if I don't have Improved Plasma Grenades, I probably won't build Avengers; no Focus Shield, no Falcon; no Improved Skimming, then I only make Shadow Spectres if desperate; no Ranger Focus, no Rangers. Rift Blast so Wraithguard aren't just an AP power sink. If Reaper Focus is changed to give a slow, then it will become another doctrine which is essentially a condition to making a unit useful.
Meanwhile, doctrines for the other factions serve as additional improvements to already potent units - look at the Flamer doctrine, for example. This is why most high level SM players I play against use some combination of Emperor's Influence and Rapid Drop Pods so that their global abilities are made more potent, giving them the versatility that Eldar players don't get, because they are forced to choose which units they would like to be viable before the match even begins, giving their opponent a heads up as to what to expect.
This is why I propose that Eldar get conditional abilities for their units that do not require doctrine choices, but do require their units to be under the effect of Battle Focus i.e. having some shields. For Eldar units to have unique abilities while their shields are up is flavourful and fits with their 'hit and run' strategy; it means that abilities such as the Banshee silence and Reaper slow wouldn't require a doctrine slot. Since the faction is pretty weak as it stands, I doubt adding conditional abilities to their already pretty expensive units could make them too strong, especially since many of these abilities are used by players already as doctrines; rather, it would simply give the faction more viability.
Like how Orks get special abilities after looting Scrap, Eldar units should gain abilities when under the effects of Battle Focus. This is fair because it gives a viable counterplay to the opponent, i.e. taking out shields of dangerous units, while also giving Eldar line units some viability without using doctrines. In a similar way, granting abilities from scrap is fair for Orks because there is an economic element to it and a counterplay, in taking out Waagh towers.
The abilities should be converted from some of the Eldar's current doctrines, such as Improved Scream, Improved Skimming, Ranger Focus, etc; these doctrines could be replaced by abilities that further enhance the abilities or change the conditions under which they are activated, or both.
Currently, Eldar's factional abilities are Fleet of Foot, and their shield mechanic. The former is useful, but only in combination with strong units with plenty of utility provided by doctrines at present. The latter is a double edged sword, that means while Eldar don't lose models during combat, also means that they can't fight while reinforcing or recover as quickly after battles. This is why I think that Battle Focus should confer special abilities to the units that have it as well as simply being a shield.
Please, Relic, instead of buffing the strongest faction without thinking, consider how it is that you plan to revamp the faction which has been nerfed to the ground since release and which is only playable for a small number of players who are willing to lose games for a very long time in order to compete while playing a faction that appears to have been designed to be 'hard mode' with a lot of weaknesses, complexities, and very few strengths (again look at Dark Reapers for an example of this). You've said that you don't want to mess with the base stats of the units, I think that is fair, but I also think that most of the potency comes of Eldar line units comes from the use you make of the doctrines you can equip for them, and I think this needs to be changed.