Eiichiro Oda has introduced fans to tons of different characters with potentially interesting stories throughout One Piece's 23 year long run, but unfortunately many of the characters do most of their development off screen because we are meant to follow Luffy and the Straw Hats' story for the most part. This means that many of the standouts are left unexplored simply because there has not been the time to do so. Characters like Luffy's grandfather, Monkey D. Garp, who needs his own arc as soon as possible because he's got a lot of cool back story we have yet to see.
Garp was already an interesting character given that he was a high ranking Marine official whose son went on to lead the Revolutionary Army and grandson is now stirring up a ton of trouble as one of the members of the Worst Generation. Even with all of this happening, however, Garp never quite loses his position in the Marines nor does he accept higher promotions either. It'd be great to see this.
Without giving too much away for those who might not be completely caught up with the manga run of the series, the explanation for Garp's high position in the Marines is revealed to be such a great feat that he essentially became the Marine legend we currently known him as in the series. This explanation also reveals that he once fought side by side with Gol D. Roger himself, and it's why Garp never really has to worry about his position in the Navy.
But we need to see this happen. Eiichiro Oda has taken time in the past to flesh out some of the more important back stories in the series with extended flashbacks, and the same could be done here for a full Garp arc. We need to see more of his younger days in the Navy, or at the very least, we need to see more of his time growing up with his son who eventually fights against him and the grandson who eventually becomes a pirate.
The main reason Garp needs his own arc, however, is because we honestly don't know enough about what makes him really tick. How does he feel about his family's current pirate life? Why does he stay with the Navy despite clearly aligning with a few pirate ideals? What makes Garp...Garp? A full arc focusing on his character or at least showing more of his own past would go a long way.
But what do you think? Do you want to see an arc focusing on Garp? Would it be better to see his past or what he's doing while Luffy is tearing his way through Wano? What character would you want to see an arc for instead? Let us know your thoughts in the comments or you can even reach out to me directly about all things animated and other cool stuff @Valdezology on Twitter!