so the mystic arcanum feature is kinda neat, but i feel like it is not that well incorperated into the warlock class and is instead just something that is kinda there, as an sort of much weaker form of high level spell slots other spellcasters get. And so to solve this problem, i suggest these eldrich invocations that let you do more things with your arcanum, that also give the warlock some spells that they should have had from the start

call for aid
requirements: pact of the chain feature, 12th level 
you learn the spell planar ally as an arcanum spell, and it does not count against the number of arcanum spells you know.

master of magic
requirements: 12th level
on your turn when you are casting a spell using an warlock spell slot, you may expend one use of one of your arcanum spells. When you do so the warlock spell slot is treated as being the same level as the arcanum spell expended. (in other words an warlock with this invocation can cast infernal calling as an 6th, 7th, 8th or 9th level spell)

tome of infinite secrets
requirements: 12th level, pact of the tome feature
you learn how to sacrifice arcane spell power in exchange for temporary greater comprehension of the arcane secrets within your book of shadows. as an action when you are holding your book of shadows, you may expend one use of your highest level arcanum spell. When you do so, choose a single spell from the wizard spell list of 5th level or lower. For the next hour, or until you lose your book of shadows, you know the chosen spell, and it is treated as a warlock spell for you.

wish granter
requirements: 18th level
Your patron bestows oppon you the ability to make wishes come true. as an action, you may expend the use of your 9th level arcanum to cast the wish spell, and when cast in this manner you can treat one willing creature you touch as the caster of the spell. Once you have cast the wish spell using this invocation, you cannot do so again until five days have passed.