So I've been thinking about how to make half-caster progression more palatable for those who liked pact magic. Here is my thought.
Right now, the warlock gets a free "phantom" spell slot. It's hidden in your Patron and it gives you a free cast of any of your patron spells. You only get one, and once you use a free cast for any patron spell, you're done for the day. (Making it worthless to use it on anything but your highlest level patron spells).
Let's eliminate that.
Instead, let's give them a single "Mystic Arcanum" spell slot. It scales at the same rate as a full caster (2nd level at 3rd, 3rd level at 5th, 4th level at 7th, etc). You can prepare a single Arcane spell in it, and it recovers on a short rest. That way, you always have a boom at the highest level, and it will augment your collection of lower-level utility magic.
For example: a 10th level caster prepares 9 spells of 1st-3rd level and one 5th level spell. They can cast the lower spells with their regular spell slots, but the 5th level spell with their special MA splot. Their regular spells come back on long rest, but the MA comes back on a short or long. When they level to 11th level, they MA slot becomes a 6th level slot and they prep a 6th level spell (or any spell upcasted to 6th) in it instead.
Pros:
Brings back some of the boom of pact magic while keeping lower-level spell slots available.
No MA Invocation tax
Cons:
Far more complicated
Potential power/balance issues.