How about an entire combat system based around customizable stuff? That would make things more interesting and varied, while appealing to something more than the vehicle nuts out there.
So, you can throw together cars, tanks, blimps, planes, boats and submarines, but you can also make small arms. Wonky swords, rifles, pistols, machine guns, crossbows, or any combination thereof. To make customization all-encompassing, allow a degree of terrain deformation and building creation, with a focus on creating your own floating mansion, which can generate profit from renting out rooms to PCs, NPCs, and store owners and give you a safe place to store supplies and fashion new creations. If a group of players work together, they could make a stronghold. A concentrated community effort could even result in a giant floating city, with different players contributing and owning the spaces that they create themselves or buy from other players. Having two or more distinct factions would encourage cooperative building.
Allow all the building elements to be combined, such as having a floating house that's propelled by rockets and armed with mounted gatling sword-guns, each element you assembled yourself, and we'd have the best game ever.
A Steam Punk-ish setting would probably work best with the invention/creation aspect of the game - And there's a notable lack of good Steam Punk games out there.