In no particular order...

A huge manifested True Demon that wasn't a big old snake, play with some Godzilla or King Kong tropes.

Some other mystic humans, other witches and psychics (though Angel ended up doing that).

A demon that was a nod to 1980's teen slasher films.

Future versions of Buffy and the Scoobies, sent back in time by magic.

A glimpse at what happened to Tara post mortem (higher being? simple ghost?)

Better looking werewolvesw (which is something else we got on Angel).

Other agents of the Powers That Be (also Angel).

Some kind of otherworldly court. Seelie Court, Duke of Hell, something inbetween.

More Gods. There was Glory and a big floating head calling itself Osiris, but folks like Thor or Hermes would be fun.

Buttercup the Vampire Unicorn. Just because.