as far as i know mal'ganis hasn't shown himself since the initial invasion on the broken shore. he plays an imporant part of lore and and there is simply no way blizzard just forgot about him. during the wc3 campaign, mal'ganis serves the lich king and succeeds in his task to turn arthas to darkness, but is betrayed and killed soon after.
i could imagine that mal'ganis has returned to azeroth seeking vengeance upon the lich king and is plotting somewhere behind the scenes slowly gaining more influence, likely disguised as someone else. i don't think that this someone is a major lore character like sylvanas or anduin because i that would make for a bad narrative and also i guess he must be able to defeat the victim in order to disguise himself as it.
he is more likely to pick a character that is not in the spotlight, but still has an influential role to someone with power, like some kind of advisor. either that, or a character that has been newly (re)introduced to the lore and is quickly gaining stature of some kind, for example as an oppressor to an authority (desolate council?).
previous lore indicates that dreadlords usually interract with the human or undead faction so that may be a good starting point for speculation. i don't think there has been any notable remarks from any charcater during legion in regards to the lich king, no? perhaps in the some of the order hall campaigns?
i know that i will definitely keep one eye open during battle for azeorth for characters that mention the lich king in some way or another.
so what do you guys think?