The issue for development of Thomas character would be to grant him his wish and not make him a Vampire, but a plain vanilla mortal no. Make him an Avatar of Erus (Eros, Cupid) the Etruscan god of love. Have him feed on and propagate love rather lifeforce, evolve from hunter gatherer to farmer, keeping his strength and speed and durability but being linked to something like Cupid’s Bow, a mythological artefact. Oh and the Sword of Love.
We only know the White Court are Etruscan in origin, we have no idea how they came into being, what they were originally. Maybe in Twelve Months Harry will find this out. Perhaps when he sits down and watches “The Father of the Bride” with Papa Raith. Thomas becomes an archetype for the evolution of much of the White Court, their redemption arc.
Foreshadowing, in Grave Peril, Thomas is effectively portraying Erus when we meet him. In Changes he is designated as Legolas, the beautiful youthful archer.
Jim has known Thomas’ story arc since Grave Peril I don’t believe him when he says he doesn’t know. He knows, it’s just how to gets there.