Custom IP addresses are probably not a feature that could be added, given that Linode has specific blocks of IPV4 and IPV6 addresses for use by customers. You can't choose your own IP address out of these blocks, and each data center has specific blocks of IP addresses assigned to it for IPV4 and IPV6.
The closest you could come to a custom IP address would be allocating a /64 to your Linode, a network block of IPV6 addresses. You could then use any IPV6 address within that range to give your Linode a custom address, beginning with your /64 prefix and ending with anything you'd like that's a valid standard for the IPV6 address format. As far as any custom IPV4 addresses, I doubt this will ever be possible, by any VPS provider. I know of none that does such a thing, unless you're using your own private VLAN, or something similar, in which case, you could use any IP address within a private IPV4 block reserved for local area networks. It wouldn't be a public IPV4 address for the internet in this case, though, just something to use privately between particular Linodes in one particular data center.