I have to tell you, when I see GPL license on a game, I see the good intention you have. Have you considered changing the license of Wesnoth to MIT so commercial game developers can use their time and effort on components can port them to their own projects too? I can't justify investing hundreds of hours programming new components a project (even one I'm passionate about) that a later derivative can't be used by my…
Regardless of the merits or lack thereof of changing the license, it'd probably present an even larger problem, since they don't seem to ask for copyright assignment and they have 100+ contributors, all of whom would have to consent to the new license or have their code thrown out and cleanly reimplemented.
As a stop gap - they can implement a contributor clause starting immediately. They can also say future commits post-[ref] are until the prior code is sunsetted.
Typically there is an uppercrust where a very high percentage of the contributions lay. I haven't looked at the code (played the game all the time back in HS) but I'd bet you 90% of the code contributions are with the top 5 committers.