On the subject of Chosen Undead's moveset potential...
When it comes to moveset potential regarding Chosen Undead, aside from the token Sword and Shield, the Chosen Undead will heavily rely on Pyromancy, Sorcery and Miracles for their Specials, much like Hero with their spells. In Dark Souls, there was always a move from each set that kept going into the next stronger move alongside a whole set of spells that gave different effects. Heck, there is a Pyromancy spell that pretty much acts like Hero's Metal Form, except they can actually move.
That said, Sorcery brought out the nanomachines in Dark Souls 2 and 3 when it came to the one spell that would get a stronger version or a variation of the move. If you have Robin with 4 versions of Thunder, Chosen Undead's close-to-home Sorcery special would have frickin' 16 of them. Probably even higher.
For a recovery move, they will have to be original on which move they choose for the recovery move. In the list of Miracles, there is Homeward, but it's more like Zoom as it teleports you to the last Bonfire you visited. But there are a few spells, like Combustion, that can cause an explosion for a hypothetical upward momentum recovery.
If we go into spoiler territory, I would not be surprised if Sunlight Spear was one of their specials, not to mention it appears in all 3 games as a Miracle. Sunlight Spear basically has Chosen Undead throw a giant spear of lightning at their foe. If you count on "how they use it in Dark Souls 3", it can also cause shockwaves when it hits a surface or they can throw it straight to the ground to cause a shockwave.
So something that can be done with that is that when you use Sunlight Spear, you can "aim" it in the brief time it charges up and it will go into the direction the control stick aims at. Aim it downward to have them throw it at the ground for a shockwave and any surface it hits when thrown will cause a shockwave.
If they want to be a bit more creative with Chosen Undead's regular moveset, there are a couple Sorcery and Miracle spells that have the player attack with magic materialised weapons. Sorcery that has the player attack with a giant Soul Greatsword, a Miracle that has the player attack with a frickin' Scythe(called the Lifehunt Scythe) and as mentioned, Lightning and Sunlight Spears.
As for the Final Smash....well, there are plenty of Pyromancy, Miracles and Sorcery spells that they can use that practically rains havoc upon the battlefield, like Gravelord Sword Dance, Firestorm/Fire Tempest/Chaos Storm, Divine Pillars of Light. But another thing that they can also use for the Final Smash is activating the "Final Bonfire", one of the Endings of Dark Souls 1, much like how Banjo has the Jinjonator. Or they could also have the Player summon Phantoms for the Final Smash, summoning phantom characters from the games, like Solaire of Astora, for a team attack.
On paper, Chosen Undead would be very very similar in concept to Hero, even if they make some of the spells they use different from Hero's spells. But I actually think I know something that can hugely differentiate them from Hero and other characters. The Parry mechanic.
The Parry mechanic in Dark Souls is one of the most remembered features of Dark Souls due to how frickin' satisfying it is to successfully land them, especially against tough foes and bosses. But as we know, there is a Parry mechanic in Smash for all characters. When the Shield button is pressed at the right time when an opponent's attack connects, you will flash, take no damage and you can attack quickly out of it.
But how about this for Chosen Undead's Parry Mechanic implemented in Smash? When you quickly press the Shield button for a parry, the Chosen Undead will do a Parry Shield bash in the direction they are facing. If the Chosen Undead successfully parries an attack, the foe is suddenly stunned for a second or a second and a half. During that period, the player playing the Chosen Undead will get a Button Prompt to press the Special Button. The player just needs to press the Special button with no direction input, like a Final Smash.
If they succeed at pressing the input, the Chosen Undead will do a Parry attack where the Chosen Undead will thrust their sword into the foe, dealing alot of damage to the foe and sending them flying. During this period, both the Chosen Undead and foe will not be hit by any attacks, meaning that other opponents cannot disrupt this, referring to the invincibility frames the Chosen Undead gets when doing a successful Parry Attack. The only requirements for this to happen is that the Chosen Undead MUST be facing the character they are parrying, meaning "back parries" will not activate and it does not activate from projectiles.