for sure, one custom i REALLY like for an Aeldari gunline is the "Swift Strike" and you can combine it with "Masterful Shots" or "Hunter of Ancient Relics", but the Swift Strike is essential, especially with Heavy Weapons.
The fact that you can than advance and then shoot your heavy weapons with no penalty is just insane.
Also nice with Battle Focus
As for units, i have nothing at the moment to add, except the Wraithguard, War Walker, Wraithlord etc...
Dark Reapers are so much better than i thought at first, and the Dire Avengers are also really cool to play.
However, i'm not really familiar with the Harlequin :/