Dracolich Phylacteries (MM P.84): (Emphasis my own)
(...)If a dracolich's physical form is ever destroyed, its spirit returns to the gem as long as the two are on the same plane. If the gem comes into contact with another dragon's corpse, the dracolich's spirit can take possession of that corpse to become a new dracolich.
Recently as I was reminded of this line, I realized that 1. the dracolich is a lich, and thus is able to become a warlock patron, 2. While the monster manual states only a true adult/ancient dragon might become a dracolich, this entry does not put such limitation to the corpse used, and 3. a dracolich's phylactery sounds like an arcane focus... This, in turn, lead me down the path to creating a Kobold Undead Warlock with a pseudodracolich patron... With the dracolich having been dormant so long, it jumped into the tiny corpse before realizing it.
Initially, I thought this would be perfect as the Pact of the Chain would represent the master, but as I used Xanathar's Guide and Fizban's Treasury to develop the characters, it painted a clear picture of an ancient dragon who turned to lichdom in hopes to "find the one who can inherit my library," making the Pact of the Tome a clear winner.
I've read many posts here speaking about the difference between a Find Familiar and a Variant: Familiar, as well as those confirming that only the Pact of the Chain can summon a pseudodragon to fight by your side, but I am finding it hard not to picture the kobold seeking wisdom and the "Help" action from the tiny skeleton on his shoulder, and the Find Familiar spell seems to be a great way to balance the reviving of a lich in-game.
So while I can see that it is ultimately up to a DM to decide if this is possible or not, I was wondering if there would be some unforeseen unbalancing to have a pseudodragon familiar variant (stat block) as my patron and use the Book of Shadows' Find Familiar spell to revive it?