Have you thought about licensing future additions to WordPress under AGPL? I believe it can be done [1]. This will disallow private forks and require companies to publish any changes they make.
It would disallow private forks of WordPress (require them to share the modifications) but I don't know whether WPEngine and other hosts have any private modifications or they all use stock WordPress.
photomatt|5 months ago
misnome|5 months ago
rustc|5 months ago