More glyphs for re-skins of the base 5 would be great - and pretty simple to implement.
There are lots and lots of options already in game: Satyrs, Dark Felstalker, Dark Hound, Imp Mothers, Nathrezim, Aranasi, Jailers, Inquisitors etc, etc. Personally - I’d love to be able to have my old, pre-legion Wrathguard back.
EDIT: But…I’d rather be able to permanently enslave demons. There is a brilliant old thread about that here: