Since Rockman 2, the skull has been used as Wily Castles' signature feature. Since Rockman 4, the skull has extended it's appearance into his signature super war mecha, the Wily Machine Series. Later editions of the Wily escape vehicle would also follow suit and incorporate skulls. In Rockman 8's artwork, Dr. Wily even sports a Skull belt buckle. With such a "skull fetish", why isn't Skullman Dr. Wily's main robot and Rockman's rival?

     I think Skullman would make a great rival character for Rockman. I would even say even much better than Forte. Not that I have anything particularly against Forte, but Skullman has so many shining attributes for the role of rival. Aside from the skulls:

1. His name format is a better approximation as a mirror to Rockman's. Rockman's name is a construct of Dr. Right's love for Rock & Roll music, combined with the suffix he puts on his humanoid robots "man". We can say that the same construct for Skullman's name exist.
2. His battle abilities are already similar to Rockman, in that "non gimmicks warrior" line. Unlike other Boss Characters, this puts him more in line of a rival fighter. Just give him some replica of a weapon change system and a speed burst move like the slide and he's ready to go.
3. The overall design style and complexity density are much closer to Rockman. This is probably a result of the time he was made. Forte was designed way late into the SFC days, so his design looks a lot closer to someone you would find in the Rockman X series.
4. And heck, even Rockman Production staff loves Skullman. Hayato Kaji, one of the art designers for the Rockman series has remarked that during Rockman 4's production, an entire stage that was in the works for the game was totally scratched just so they could fit Skullman into the game. That has to be worth something, right? (though to be fair, Hayato is also the creator of Forte)