But between uses like setting up a personal VPN with a clean IP or just the cool idea of having a personal IPv4 address or IPv6 block... I think it would be a viable, if rather small and niche, business.
There are effectively no more clean IPv4 addresses, you'd have to buy addresses that had previously been used.
Anyone can get IPv6 addresses even those whose ISP sucks via tunnelbroker.net (aka Hurricane Electric) which will provide a single address, also a /64 and/or a /48. Of course they are generally blocked by streaming services since they are a form of VPN and thus the endpoint might be anywhere.
What are the terms of assignment via Hurricane Electric? Can they take it away? Do they only allow BGP advertisement to their sites or can I still bring the IP elsewhere?
I still think there is an opportunity for niche needs here.
An HE IPv6 tunnel is as permanent as you like, but they reserve the right to phase out the terminal you are using which sometimes means your prefix would change, and they expire unused tunnels periodically. IPv6 has builtin handling of prefix changes though it does not deal with related DNS updates, which you'd have to arrange.
An HE IPv6 assignment is from their allocation so you'd call that PA not PI, i.e., you can't take them elsewhere. To get addresses of your own you would need to apply to an LIR or RIR for an allocation -- generally easy to get a /48 without any/much documentation with a /40 generally requiring documentation but that's not free (250/yr for an ARIN allocation).
Hiding bots and other users among the huge Comcast & Spectrum IP blocks makes it harder for media companies to block them. It’s just how do you legitimately buy a bunch of consumer connections, or get users to install software to share their consumer connection, hence the NordVPN clients I assume.
Granted, abuse can occur. But assume US-based with basic KYC or something so that most IPs might genuinely be nerds like me using it as their IP.