It has been queried before and basically there are 2 schools of thought.
Currently people post their project threads in the forum for their type of car. It could be kept that way as it keeps things logical and if you're into 8P A3s for example you can find all the tuning and modding for them in one place.
You could of course keep all those forums but just have a thread dedicated to projects as you have suggested, but that could cause a dilution of content. If I have just done a big thing to my car I want everyone with that same type to know about it. If I put it in the project forum it could be missed.
This isn't really a modding forum as such so most people just hang around in the forums for their specific car and don't look elsewhere too much.
It's not a bad idea of course to have that dedicated modding forum and it's not been ruled out altogether. There has been a big update recently and FactionOne (the Admin) is a hugely busy guy, so all major mods to the site have been put on hold for the moment.