> I guess it means the Play store will be the only way to install an app
No, non-Play stores will still work, but developers will need to register a developer account with Google that is tied to some real identity. They already need to do this to distribute through the Play store, but now it'll apply regardless.
This is to make it harder for scam apps to churn app signatures. Kind of like requiring code-signing, but with only one CA.
> That users won't be able to install what they want
No, sideloading will still work, but it won't work if the APK isn't signed by someone in the Google developer registry.
> and that they would need a google account to install apps
Nope.
> That app developers have to go through google to distribute their apps, with identity verification etc.
They don't need to distribute through Google, but they will need to be involved with Google and do identity verification.
> However, I'm also reading here and there that it is a threat to alternative ROMs. To me it sounds at the contrary as an amazing opportunity, as they can strip this verification and be the only truly open Android, or am I missing something?
You're being misinformed. They won't even need to strip the verification. The verification is only for certified Android -- OEMs that partner with Google. Custom ROMs and the OEMs that aren't certified (Amazon, some Chinese manufacturers) won't have verification.
The target audience for verification and who would ever use a custom ROM has basically zero overlap.