I'd say Pact of the Tome is about Knowledge, while pact of the quill is about Creating. A book is a repository, while a quill a tool of creation and change, only relevant at the moment of creation and alteration.
I'd give the Pact of the Quill the ability to create "real illusions" similar to the school of illusion.
I'd give it the ability to influence others magically via charm effects - to rewrite their stories.
I'd consider some of the dunamancy spells from the Critical Role book as they deal with changing reality in some places.
I'd give them expertise in either Intimidation or Persuasion, and give them the Performance skill for free.
I'd give them the ability to learn a tool kit as an invocation, and craft using it in a single round rather than taking the normal time frame.
If Blade is warrior lock, Chain is summoner lock, and Tome is ritual lock...
Quill would be a scroll lock.
If there is a magic book already, a magic quill would not writing on books. It would be signing contracts and creating scrolls.
A Quilllock would make temporary magic scrolls to cast their patron's spells. Invocations would let the quill lock get access to all sorts of message and writing spells.
The Warlock gains a quill the style and looks defined by the player when first received. The Warlock can use their quill to write a spell scroll containing any 1 spell up to a level one less than the highest they can cast min. Level 1. This scroll lasts until it is used or another scroll is written when immediately the first scroll will disappear. the scroll can be given to anyone but only one scroll can exist at a time. The quill can also be used as a spell casting focus and if destroyed can be replaced via an ritual done over the period of a long rest.