If I make a VPN connection to a server (say myvpn.com; assume this server is not behind any router/firewall) via a TUN device and open a port (say 5555), will someone else be able to connect to me via myvpn.com:5555? If not, is there a tunneling software that does exactly this in a transparent manner? i.e. run any TCP/UDP-based server instance behind a router without NAT using another remote server.
I'm not sure I understood the idea correctly, but if you want clients connecting to myvpn.com:5555 actually get connected to your host, you can try SSH port forwarding (-L option):