Adding a big +1 to this as this should be achievable with wire guard. Another nice to have would be the Tailscale funnel type feature to option up services to the internet.

Your idea of adding peer-to-peer mesh networking to Proton VPN is excellent and very forward-thinking.
Providing private IPs and enabling NAT traversal would make device-to-device communication seamless, much like a local network.
This would greatly enhance use cases like cloud gaming, remote development, and syncing across devices.
Optional routing via Proton servers adds flexibility and reliability when direct connections aren’t possible. https://sonmatypingexpert.com/ Overall, this feature could significantly elevate Proton VPN’s capabilities and user experience.

This seems redundant in my opinion. Why not just get a solution like Tailscale then?

Why deform Proton into something it isn't, when there are free experienced/advanced and highly specialized solutions already out there being maintaned?

One of the most important features for me to access my devices (desktop mostly) while I'm away since my ISP forces me to use its router and I don't have a fixed IP

would love this, as id then be able to cancel my mullvad sub, since its kinda annoying that i pay for 2 vpns but i only get to really use one (i have mullvad for tailscale and i have proton vpn)

As you said, this is already implemented entities such as Tailscale. The purpose behind protons vpn is different (evading geo restriction / censorship, privacy, anonymity, and security from malicious networks)

I would love to see this so I could keep my devices synced with notifications and other things. having an optional solution for ZeroTier would be very helpful especially on Android where I can't run two separate VPNs simultaneously