Hi there, I'm posting here because it's more underground compared to Reddit (since I think that the dev wouldn't like it to get mainstream attention).
There is an open-source WC3 engine called Warsmash (made using LibGDX): https://github.com/Retera/WarsmashModEngine
( Discord server: https://discord.com/invite/ucjftZ7x7H )
I thought I might share it here, if anyone is interested in participating in its development. There isn't a Flatpak version yet and it also misses some features e.g.
If the author really wants Blizzard to use his code then he doesn't have much of a choice. Although since it has a GPL library the whole project is distributed under the GPL.
As the copyright owner you can do whatever you want. You can make your code gpl and give (written) permission to only blizzard to use the code under a different license.
You'd need a CLA for potential contributors though and I have no idea what courts think about it (not a lawyer, not legal advice).