1.
LOOK AT HIM
I would think the appeal of a Liche in coal black Centurion armor is self-evident, but maybe not. He has one of the most badass designs of all of Nintendo's villains, and fits a classic video game archetype that no other Smash character really does: the undead monster. Sure, maybe his corpse-like appearance is a little much for E10+, but he could be redesigned as a skeleton or simply a slightly less frightening zombie if need be. Regardless, Pious Augustus is METAL. See his staff? IT'S A SPINE.
2.
His game deserves more attention
This thread has solidified this point for me. Sure, it's "obscure", but if Smash speculation has caused people to seek out and play
For Whom the Frog Bell Tolls and
Joy Mecha Fight, surely people can make time for a game that's actually good, right? Eternal Darkness may be obscure to the average Nintendo fan, but it is NOT obscure to survival horror fans, and is often considered one of the three undisputed masterpieces of the genre, alongside
Silent Hill 2 and
REmake, while adopting a much more unique, fun, and dare I say Nintendo-esque angle on the genre than those other games. It has also been very influential. Ever heard of
Amnesia: The Dark Descent, the indie title that basically brought about a Survival Horror renaissance a few years ago? Its central mechanic was a "sanity meter" that lowered whenever you made contact with enemies, causing hallucinations and interface screw if it got too low. That mechanic was ripped straight from Eternal Darkness. Nintendo even had a patent on it.
Call of Duty: Modern Warfare shocked players when their character died at the end of one of the single player chapters. It was such a lauded twist at the time that they've basically repeated it in every CoD game since. Eternal Darkness did it waaaay before CoD. Smash has brought attention to many lesser known Nintendo classics, and Eternal Darkness is among the best of them. This is a game that begs for a sequel, but it'll never happen if Nintendo's rank and file fans don't give the original a shot.
3.
He is the most deserving rep for his game
Eternal Darkness has 12 separate playable characters across as many chapters. Pious is not only one of these playable characters, but as the game's main antagonist, is the only character to appear in every chapter. Alexandra Roivas, the ostensible "main" character, doesn't even encounter any enemies until the end of the game. For the vast majority of the game, she is just the person you play as in the hub world between chapters, solving puzzles and getting jump scared for about five minutes before you jump into the next level. People who insist that she is the only deserving Smash rep obviously haven't played the game, or at least lack imagination. She is by far the character with the least to offer in terms of move set potential and aesthetic uniqueness, and her role in the story is actually far less significant than Pious'. And while it is unprecedented for the "hero" to not be the first playable character from their series (unless you count Pikachu getting in two games before "Red"), Smash has never featured a character from a Horror franchise. When it comes to horror, the "face" of the series is nearly always the monster. In movies: Freddy Krueger, Jason Vorhees, Michael Myers, Pinhead, Chucky, the Xenomorph, Samara... In games, Pyramid Head and the nurses, Alma, SHODAN, the zombies from Resident Evil... These are the characters that stick in your memory, that come to mind first when you think of their franchises, not the heroes. The same goes for Pious Augustus and Eternal Darkness.
4.
He could bring something new to the table
Eternal Darkness is unique among Survival Horror games due to the depth of its combat. The player cannot only use a variety of ranged and melee weapons to target specific body parts and incapacitate enemies without killing them, but can cast a wide variety of defensive and offensive spells that use a Rock-Paper-Scissors dynamic based on the "elemental" affinities of the three major elder gods: Magick, Matter, and Madness. Among the spells are spells for enhancing the damage of normal weaponry, healing, shields, barriers, AoE damage spells, "fireballs", and last but not least, summoning and even possessing the game's various enemies in order to aid you in battle. This system ends up feeling fun and very tactical, and the game often throws many enemies at you at once, necessitating divide and conquer strategies that make heavy use of spells. In universe, Pious Augustus is the undisputed master of this "magick" system, and as the final boss can cast more powerful versions of every spell that the player can. On top of that, he also has his enchanted Roman gladius: a short sword (a swordfighting style not really seen in Smash yet), and a big bone staff. If Sakurai remained faithful to the nature of ED's combat when putting together Pious' moveset, he would be largely based around stage control, creating barriers and summoning Zombies to keep his opponent busy while he charges up powerful offensive spells or gives himself buffs, in some ways playing a bit like a MOBA character. This would make him not only a very unique character in Smash, but in fighting games in general. Some may resist the idea of such a turtley defense-oriented character, but I think that if balanced well, it could make for a very unique fighter with lots of options.