Ever since Legion, the roster of demons has expanded.
WIth that being said, isn't it time for warlocks - like hunters - to be able to permanently enslave/summon a bigger variety of demons.
Demons such as:
Wyrmtongue
Vilefiends
Fel lords
Inquisitors
A pack of Felstalkers
Felbat
..or bring back the Doomguards
Aranasi
or mayhaps Pitlords
What do you think?