It's a nice idea for the modding community, having a sort of online database that people could access. Not everyone likes going through forums or other fansite databases to find mods or custom content. However, despite any engine changes that would need to be made... small or not... there's still then the server hosting for the database and such which add to the bottomline. What those sorts of things would cost, I'm not sure... and isn't really necessary to know in my opinion.
I think the biggest problem with having a 'in-game ship creator' that would be accessible and upgradable in the course of the regular game is the lore issue. The ships that are out there now are those that different groups have access to, or access to the blueprints that make them. Correct me if I'm wrong, but new ships weren't being developed and made because much of the capabilities to do so just aren't supposed to be there. So, adding in such facilities to create your own that no other faction will use (it's your unique build) just seems to break the feeling of what the universe is going to be like.
And on the minor end, there would be the balancing issue. That's why ships added in via mods is fine since you know you're modding your game beyond what was intended in testing and design by the developers. Having something accessible in the regular gameplay experience to me would feel like all the work to balance everything would be pushed aside for people to create some ultimate design that can devastate anything it comes across. (and yes I know the OP suggested limitations so it wouldn't, I'm sure)
Anyways, I'm for an online or in-game database system for ships and/or mods. Not sure I like the idea of an in-game ship customizer/creator.
