+1 to MihoshiK's suggestion, been itching to draw a superprop for a while now
A few further suggestions of my own:
FD:
Quite The Package - Create a enhanced armor protection package for an existing (older) vehicle; this can be an open-ended challenge for anything from tanks to APCs to cars
(hardkill/softkill/era/etc). Would need to show before/after images I think
Pressed Into Service: Design a modification for an aircraft whose role(s) wasn't originally intended. (ex: passenger aircraft > bomber, fighter > night fighter, fast bomber > recon aircraft).
SB:
Makeover: Design a conversion for an ship either from irl or as it has existed in your AU.
What Coulda Been: Come up with a preliminary design study/never were of an AU ship.
SoBu:
Plot Armor: Design some body armor sets as you see fit (any timeline).
GB:
Damn Shawty: Design a shotgun, seems simple enough right?
First Person Shooter: Come up with a fun/wacky gun design for a fictional main character.