Infernal, abyss, fathomless, undying, unseelie are all unspecific in the type of entity you're pacted to - so long as they fit within the name. There are countless powerful devils, demons, creatures of the depths, unnaturally eternal creatures, and fey that you could model your roleplay around with the other pacts.
Night hag pact is a very thematically interesting path for a warlock to take, but by the name alone suggests your only patron option is a night hag.
Simply suggesting a name change to offer more RP options for night hag pacts by renaming it to something like "Nightmare pact" that opens roleplayers up to some of the other terrors that go bump in the night. Bastellus, creatures from the Night Parade, other creature types that could occupy your nightmares. This is purely a change for roleplay as I still feel the night hag projection summon is the peak of nightmare conjuring and is the most powerful summon a warlock could access through their pact powers.
Note: Could be something other than "nightmare pact", that's just what I dreamed up. It could be called something like "covetous pact" to suggest other hag types and creatures that influence your mind to get what they want.