Personally, I tend to make things for 2 reasons: I want one for personal use, and #2 I think I can sell them because we're the only one that makes them...
There's tons of helmets, swords, and shoulder pads, but what else would you like to see on the 3rd party 3D printing market?
Here's my short list:
TechMarine Kit (modular backpack, Omnissian axe, belt buckle, maybe a helmet, shoulder pad)
Classic Space Marine greebles
DIY Servo Skull
Man-Portable Las Cannon (I haven't seen one of these yet...)
A more accurate Deathwatch shoulder pad.
Jet Bike
Although I'm pretty sure a wheeled attack bike would sell better...but I have a soft spot for these old models.
Dreadnought
Yes, we're making a 3D printed McFarlane-sized Dreadnought. It's gonna happen.
I would put Librarian Kit up on the list, but we just finished the prototype, so it's no longer a "wish". When we get the TechMarine kit finished, I'll have my Chaplain, Librarian and TechMarine...ahh...bliss.
Are you a McFarlane Primaris fan? What would you wish to make your next build "Perfect" or "Over the Top"?