There are two ways you can go: you can allow total player freedom to describe their patron any way they like. In which case, it's a few paragraphs on a written biography that has no actual effect on gameplay. Solasta works this way.
Or, the DM can create and run the patrons, in which case you have story and interactivity, but limited player choice. Given a fixed amount of development time, the more patrons you have the less story each one gets; the fewer patrons, the more story.
From the point of view of a writer, the more interesting stories stem from conflict between the warlock and their patron.
Well, they can also take the same voice and assign different bodies to it.
Let's say that there would be a few unique dialogues that would be specific to Fey, Fiend and Old One.
But the basis would always be the same. This would allow for maximum story gain.
