FormerlyHemlock
Hero
So, Mystic Arcanum. That more than anything worries me. They seem rather...bad. Once per day abilities from a very limited list, hence very little customization.
They're not so bad that you wouldn't want to have them. It's nothing like being a wizard or a druid, but some of the abilities are rather good, probably as good as a feat. Mass Suggestion and Create Undead at 6th level, Forcecage at 7th, Demiplane at 8th, and Foresight or True Polymorph at 9th... hmmm, I think I just proved your point for you, didn't I? For the abilities that I've just called out, there are only four possible build permutations. (Mass Suggestion/Foresight, Mass Suggestion/True Polymorph, Create Undead/Foresight, Create Undead/True Polymorph.) In reality there are a few others that might be worth taking[1] but it winds up being pretty narrow. By the time you've had two high-level warlocks you'll probably be sick and tired of those options and itching for a change.
Who knows though whether that is a problem in actual play. Not many people seem to play to 20th level at all, let alone multiple times in the same class.
-Max
[1] If your DM lets you take Etherealness as your 8th level invocation, castable as an 8th level spell, that is quite good; and Create Undead at 8th level might be worth taking too.