Does my open source project need be Arduino based to be successful?
I think it depends who you're trying to attract. Having an Arduino derivative which can be programmed directly from the Arduino IDE will attract Arduino users.

But, in my experience, the average Arduino user is not neccessarily a skilled developer who will contribute usefully to a project. Of course, there are many expert Arduino users, but it's a device aimed at the beginner.

On the other hand, the kind of user who can get a toolchain and debugger going for ARM Cortex-M3 is also likely to be capable of fixing your bugs for you. But, there are considerably less of them.

I'd say - pick what's right for your project. If it's good, users will get involved.