Does it increase fingerprinting? I imagine there might be some non-user-agent way to determine the OS. Like with image handling or whatever.
It’s probably more unique and suspicious for a linux browser to pretend to be Windows than a Linux system disclosing itself as Linux.
The question is, what applications would run in ReactOS that wouldn’t run through Proton/Wine? Some applications require relatively obscure userspace quirks and tricks, in which case it doesn’t matter if they’re running through Linux+Wine or ReactOS+Wine if Wine doesn’t implement them.
Other than that, the rootkit anticheat that makes so many games not run on Linux are written by companies that have a near hostility towards Linux and open computing in general. If some version of their anticheat happened to work in ReactOS, they’d patch that out super fast. Same with DRM stuff.
Linux and Windows are, I think, too different conceptually to copy things from one to another. And I’m not sure what Linux-compatible innovation that ReactOS would have that hasn’t already been thought of. Performance of the Windows kernel is, at best, average.