Boundman was pretty much that.
Boundman =
Power
Snakeman = Speed
Tankman = Defense
Maybe a form that focuses on agility/mobility?
A Kangaroo inspired form maybe?
Also two of the animals that seemed to be the closest to Luffy were a tiger and a lion
, so perhaps something focused on lethality could have been a thing, although not sure how that would work with the limitations of Luffy's abilities without going into G5.