This is a list of some protocols people have talked about wanting to implement or learn more about.
Gopher
IRC
WAIS
NNTe
priv-mail
9P (or 3P)
MQTT
SCGI
BGP
MPEG-TS
OTR
XMPP
MPD
SUPDUP
State Synchronization Protocol (Mosh)
Message Posting Protocol (MPP)
NTP
ACME (Automated Certificate Management Environment)
ICB (Internet Citizen's Band)
UART
Modbus
Telehash
...