I've been working on software to modify a game's resource files, and I'm planning on releasing it in open source. I'm perfectly fine with releasing my code under gpl v3. However, I'm afraid that that would deter others from writing extensions for my software, as I have had others offer to do so - the community which I'm working with is very much afraid of open source leeches, and as a result, the majority of popular released projects are closed source. As such, I'd be more comfortable with something like BSD or MIT.
The codebase is extremely modular, and one window might consist of the end products of many different plugins. One plugin which I am working with is the work (licensed under gpl v3) of a person who I have communicated with, who was enthusiastic of me using his software. As one other person has contributed to that software, I don't think he can simply release his source to me under a different license. In the future, I'd like to have more plugins for a variety of things which would be difficult to manually implement (as doing so would require large amounts of reverse engineering).
Tl;dr: As I am in good terms with these people, is there any means for me to use their code as plugins? I would have permission, but licensing would be the issue, since others have contributed to the codebases. If so, would it be okay if i distributed their plugins with my built software (in a plugins folder)? How about placing them in the code repository and visual studio solution?
Any other advice would be appreciated.