Truly private vlans for machines that are in the same datacenter from the same account (or via an app-key style config for the vlan)...

IE: User A, Machine A can see User A, Machine B but NOT see User B, Machine A.

Private addressing of course.. This differs from the way private addressing in the DCs is currently configured in that currently User A, Machine A can see User B, machine A.

I know the default response from many would be "firewall it" or "use VPN" or "force encryption protocols".... but there are situations that this would be useful....

I don't know how many customers Linode has per datacenter, but I imagine they would probably have to be some technology other than VLAN, since there is a limit of 4096 (12-bit). MPLS has a 20-bit label giving about a million unique networks, but the licensing costs from Cisco might make that prohibitive. VXLAN gets you 16 million IDs from a 24-bit label, but that is still considered experimental. Any of these are going to drastically increase the management overhead, which makes it unlikely anytime soon.

Could it be useful? Yes, somewhat. Would it enable you to do or accomplish anything you can't do now? No.

I was thinking about this last night. My thought was a private network and allow the customer to return their public IP for a $1/mo credit. Some systems I don't wan't accessible from the Internet (DB, LDAP), but I also don't want to waste a public IP.