Based on my experience with how other game companies allow modders to extend their game, I suggest adding the following concept to the NDA (whatever it translates to in legalese):
- Any modifications to the source code you do and distribute under this agreement become copyright of Goldenhawk and may be used by Goldenhawk in any way and for any purpose, with or without permission or attribution. (although the mod author retains the right to use/distribute them).
This is to ensure that Goldenhawk is able to reintroduce good mods/fixes back into the main game without having to deal with copyright, permission, attribution, royalties, or any other grey area that could otherwise fracture the source into forks which cannot be reintegrated by Goldenhawk without being bogged down in copyright hell.
(Yes, attribution to modders is a "nice thing to do" in game credits, but it shouldn't be a legal obligation and Goldenhawk should be able to do so at their discretion, otherwise way too much drama (e.g. multiple people worked over a mod and argue about who/how should be credited).