Open Source Licensing – ‘Do Whatever You Want’ License with Attribution

Even if technically a permissive license is compatible with the GPL, sometimes explicitness pays. Do you really want some company with uber-anal lawyers to prevent its employees from using your permissively licensed code on the one in a million chance that some tiny detail of the license you chose makes it technically incompatible with the GPL (even though that's not what you intended)? If not, then unless your permissive license is very widely considered compatible with the GPL, you should dual license it.

I would say the GPL (or AGPL) is probably the best license to cover most of those points. There's no existing license that covers all 4 entirely, and it's questionable whether you can actually enforce all those points, since they don't quite add up. For example, you want people to push modifications to you, but you still want the right to determine who can redistribute the work. This can only happen if you require that all committers hand over copyright of their modifications to you. This might dissuade some potential committers.

On you're third point, you are worried about other's redistributing, but you want people to submit their changes back. With the GPL, these are effectively the same thing - because distribution of modifications can be taken back into the main branch by you anyway. It is required that anyone distributing a modification does so in source form.

As for requiring that all extensions be redistributable, this conflicts with the idea of being used commercially (assuming you mean in proprietary software. It does not restrict commercial use though, provided the source is still distributed).