Banshees

As a long time aspect cw eldar player, the first thing I thought I would cut from a combined list was the howling banshees. DE already have fast melee infantry, what could the banshees add to that.
But I'm having second thoughts, for a couple of reasons. First and biggest is the banshees mask. Denying overwatch that could weaken your wych cult units before they get into melee.
Second, wyches with shardnets solve the banshees biggest weakness, things falling back and exposing them.
I'd probably take them in supreme command with farseers or warlocks for psychic support. And maybe biker autarch for more banshee masks and joisting planes.

Sounds like a good idea to me. Wyches hold but don't kill enough while the banshees are the opposite.

While you're thinking along these lines, the other choice is to consider is souping in Harlequins to use the Silent Knife stratagem or a Haemonculus with the Vexator Mask.

The other CWE units are irreplaceable though - Doom/Jinx is MVP in all my lists, so I'll have to pick up some banshees to try out in my usual battalion detachment (typically rangers + Farseer/Warlock pair).

Haemonculus isn't going to do the same job. You want them behind a unit for protection, and they aren't fast enough to get that charge off reliably before wyches.

Banshees are fast enough they can get a turn one charge agaist the front line for 1 cp. Or charge anything not wrapped the second turn if in the webway. Theyre also better able to engage multiple units.