Playable units? Are you only referring to surface ships? Or are you happy with
any new unit.
There're mods for a human-playable
VIID and
XXIII. Here are somelinks to get you started...
http://www.subsim.com/radioroom/showthread.php?t=120420
http://www.subsim.com/radioroom/showthread.php?t=116988
And, although I know it is already available in single-player mode, there are mods that permit the
U-Flak to be taken in Multiplayer games.
I hope that helps!