The newest member of 7 Warlords

Jewelry Bonney 

A chosen pirate who has a huge prestige with a strong sense of justice would likely be the best candidate for a Warlord position . Although it has not been announced yet but obviously ,it was jewelry bonney would likely complete the 7 warlords that has been discussed by the marines in marine head quarters.
A pirate who have a bounty over 100M after being captured by the Marines should end up being imprisoned in impel down just like ace and jinbei,  but how come Jewelry Bonney is still free?
at that time , the 5 elders of the new world discussed about the vacancy of the 7 warlords position caused by jinbei's turning against the marines and resulted to imbalance of power in new world. There must be a possibility that she was offered to be one of the 7 warlords and she immediately accepted the offer.

I think her goal of joining the warlords is to revenge against the blackbeard pirates moreover , it would be an advantage for her to start her plan of revenge without being chased by the government.