Many expected that Playstation All Stars Battle Royale would be a Smash Bros. clone, but it surprisingly wasn't, and had a very, very different means of play.
Logically if Sumo Entertainment (most likely and they did hint at wanting to make one sometime) did develop a fighting game, I'm confident they'd be able to find some way that would make it stand out. The company's made two wonderful racing games, so I'm sure they could always do well in another genre if they felt the need.
For an April Fools' prank they should have announced Sonic the Fighters... Sumo Edition. Made by Sumo, and featuring all the characters as sumo wrestlers.