This sentence could easily be translated with the same syntax in English:
Dompter un djinn me doterait de pouvoirs immenses. Ce qui pourrait me servir, un de ces jours.
Taming a djinn would give me great powers. Which could be useful one of these days.
Then, it’s up to your interpretation. “Which/ce qui” can cover the whole previous sentence (taming a djinn) or more specifically the consequences of it (the great powers).
To my mind, what the writer has in mind is the powers the djinn would give them instead of the djinn specifically.