Off the top of my head I can’t think of anything that should be in OTP.
On the other hand I can think of a lot of applications and modules that should NOT be (debugger dialyzer diameter edoc eldap et eunit ftp inets megaco mnesia observer odbc snmp ssh tftp wx xmerl, to name a few).
OTP should be the small core on top of which everything else is built, nothing else. It doesn’t make any sense to have the entire world inside OTP directly.