The Reverie arc in One Piece is not considered filler, as it is part of the original manga storyline created by Eiichiro Oda. The anime adaptation faithfully follows the manga, so you can expect the Reverie arc to be important to the overall story. It's a crucial arc that sets the stage for future events in the series.
However, he is also an equally important fighter in the crew, being part of the Monster Trio, along with Zoro and Luffy, and, one of the Pirate King's Wings. While Sanji does not have a Devil Fruit, if you were to ever eat one, there are certain ones that would fit him perfectly.