Protocols for a v4 frontend

[kasperd]

I am working on a v4 frontend, which can be used by anybody who need to host a service on a machine which has a public IPv6 address but no public IPv4 address.

I wanted to hear what you guys think might be useful protocols to support on such a frontend. For now I have http and https support. I am thinking I should add SMTP and authoritative DNS support as well, though I am not sure which of those two would be most useful.

Are there any other protocols, which would be useful to support?