[lxgr]

To GPs point, though, most of these will unfortunately be dropped by most middleboxes for various reasons.

Behind a NA(P)T, you can obviously only use those protocols that the translator knows how to remap ports for.

[Hikikomori]

Can also do 1:1 NAT for IP protocols like ipsec, or your own protocol.

[lxgr]

Yes, but who else does? Network effects are important in a network.