Let's be honest, the only customization we need is more jewels.

JEWELS EVERYWHERE!

"Outrageous....truly truly...outrageous"

Good ideas
In what program did you make that? Just paint on top of the model?

Gimp 2.8 I used the craftworld codex image, and then added layers in order to customize it.

Thanks, that is the program I used to paint my image of DJPenguin in (damn I was bored that day...) so I should still have it
And yes, I will try to paint a Striking Scorpion later.

Bonus info: My current avatar was also painted in that program.

Having a read through of the codex atm, and some of the variations that could be used for customization are:
-helmet color (plus crest coloration and runes)
-pennants and sashes
-weapon color
-back banner things
-underarmor color (ex: whereas a fire dragon might normally be all orange, some only have their plates orange while their mesh is grey)
-color scheme inversion (ex: warp spider traditionally red with grey shoulders becomes grey with red shoulders)
-helmetlessness opens up the option for all sorts of fabulous hair styles
-vagazzling with spirit stones and gems
-sleeveless overcoats

no doubt at least some of this has been covered by others, just though I would do my part ;D

I think there should also be harlequinn alternate costumes for the diferent aspects

og this. so this.

I assume this might have been suggested before, but in case it haven't: I think they should let use the different variations of the Aspect Runes.
"Normal" Striking Scorpion rune:


Variations:


And the Exarch should (obviously) use the Exarch rune:



Also, I still hope they will allow us to wear wraps/bands of cloth in our craftworld's color and hopefully some with our craftworld's rune on.

The Dire Avenger is the best example of this, but the Howling Banshee (left arm) and Striking Scorpion (upper left leg).
Also, notice the Striking Scorpions right knee pad/plate, it is a non-standard version which would be a nice way of implementing customization for Eldar (though paying 10k RTP for a knee pad for a single class would be quite a lot).

I wrote a consideration for the harlequin versions

Striking Scorpion - Solitare
Dire avenger - troup master
Dark reaper - death jester
Warlock - Shadow Seer
Fire Dragon: Harlequin troupe player
Banshee Harlequin troupe player
Swooping hawk - Harlequin Troup player

I made a similar suggestion a long time ago. I said I would settle foe skins, but said pretty please with a CC on top for the subfaction.