I am not a lawyer, so I tend toward a very conservative interpretation of anything we come up with, and none of this is actual legal advice, just my understanding.
GWT is licensed/distributed under the Apache Public License 2.0, so any code contributed must be compatible with that license to let us continue to distribute as we do now. Using the GWT compiler, the JRE emulation will be transpiled and and distributed with your own code - remaining compatible with Apache matters here too, so that the legal requirements can consistently be met by developers using GWT and distributing licenses properly.
At this time the main GWT repo is covered by Google's CLA (and I suspect that even if we stop using a CLA for GWT itself, the JRE will continue to use one, since it is shared by J2CL), which has a few limitations, such as needing to certify that you authored the code, and that if you didn't that you have the rights to contribute the code and have provided all of the licensing details.
https://cla.developers.google.com/clas to read more about this.
As an example, there is code included in GWT emulation that isn't copyrighted by Google, such as some emulation for javax.validation and hibernate validation implementation. These are licensed under APLv2 also.
Beyond this, I would suggest asking about specific cases/ideas you have in mind?