A few ideas on how to expand the harem Hermione, Ginny, Gabriele, and I think Fleur all owe Harry life debts.
I've seen a few fics where a veela who owes a life debt if the person they owe the debt to is not their lifemate they become that person's slave, also some where the life debts between of witches and wizards grow larger if not paid so both Hermione and Ginny owe Harry big on theirs so have him call in the debts and since their so large he can ask for anything.
He can get Tonks through a debt that her family owe to the house of black, read a fic where this happened Mrs Tonks made a deal with Sirius's mother to barrow money to pay for potions to save Ted Tonks' life and the money wasn't paid back in a certain timeframe then Andromida Tonks' daughter would become a slave to the then Lord Black.
Another person you could would be Su Li as revenge for the way she treats Luna another one of Harry's slaves, Luna joins willingly after finding out due to her abilities like she does in Animagus Mishap.