I really really want Bonney to join the crew. No, I don't think there's too many people already. No, I don't think it's too late in the story for another person. No, I don't think she should join the revolutionary army.
Bonney is an orphaned child who dreamed of sailing the seas and going on adventures. She wanted to go to skypiea for fucks sake! One of Kuma's last wish for her is to become an "indomitable pirate". I know joining the revolutionary army would be following in her parent's footsteps, but I don't think Kuma would want that life for his daughter.
Kuma would want her to be free and to explore the world in the way she never could before. He would want her to have a loving family and the strawhats can absolutely give her that. No parent wants to pass down their responsibilities to their child.
She deserves to be as much of a normal child as she possible can. Let her join the crew and learn how to be an amazing pirate. Let her have silly little adventures with everyone. Let her eat all the delicious food she can. Let her help Nika liberate the world.
Let her be FREE.