"Ah, but it'll be difficult to create decals that fit on every ship!!!"
Possibly so.
Just give us a simple tool that allows us to mirror, scale and rotate decals and then move them around manually.
Problem solved.
Same thing with "greebles".
Instead on buying a ship-kit, I'd much rather buy a kit of random ducts, tanks, hoses and aerials that I could just manually position on my ship.
If you could scale them manually (within reason) and move them around, FDev wouldn't have to worry about making stuff fit.
It's just be up to each player to move/rotate/scale parts to fit as best they could.
For starters, though, I'd buy packs of "occupational" decals.
Most of us have ships built for exploration, mining, combat, trading, carrying passengers etc.
It'd be nice to have something to visually identify a ship's role.
Said it before but it'd be a good way for FDev to interact with the community - and generate a bit of cash - if they created a simple decal-creation tool, gave it away for free and then let players create decals which, once finished, would be submitted to FDev (automatically, by the tool) and then FDev could pick some of the best ones each month and put them up for sale for a couple of hundred Arx each.