A lot of people including myself have been talking about this in another thread. So its a good job that you made this separate thread.

One thing I need to know as I am an anime watcher only, I dont read manga so should I be expecting any spoiler in this thread. You should put some kind of alert on this thread.

Now getting back to the topic.

Like Deblas said A Musician is absolutely on top of the list for Luffy. Entertainment and Adventure what Luffy is all about.

Also do we need a male or female crew member? If another female member joins the gang then Sanji will have a hard time juggling them.

It will be hard for the new person to adjust and be comfortable in the crew. Whoever Luffy decides to pick will be a great character. Maybe he will pick someone during Skypiea arc.