[FairyKing]

I'll share a couple ideas for theoretical Zero-One forms:

Ideally this form would be the culmination of Yua breaking free from Gai's control, if that arc had been given better development. Fighting Jackal Custom, using the Fighting Jackal key in the Yaiba Scytheriser, a weapon Yua created (possibly with Naki's help) by modifying the Shotriser/Slashriser design to better suit Fighting Jackal's combat style.
"Hunt! Authorize! Kamen... Rider... Kamen... Rider... Scytherise! Customize! Fighting Jackal! Deciding the fate of battle like a Valkyrie." Its finishers are "Fighting Bite!" and "Fighting Bite Judgement!"

These forms would be if a new Magia threat required Naki and Ikazuchi to trade their Zetsumerise keys for Progrise key counterparts:
Packing Wolf for Naki (packing has a double meaning because it refers to both a wolf pack and slang for carrying a gun)
"Backup! Force Rise! Packing Wolf! BrEaK dOwN!"
Blasting Ostrich for Ikazuchi (blasting as something similar to lightning, ostrich as a still-living counterpart to a dodo, and together they make a play on "blast off")
"Rocket! Force Rise! Blasting Ostrich! BrEaK dOwN!"

In another story, we see an entirely simulated battle inside Horobi's mind, between the Sea Scorpion Magia representing the lingering malice still inside him, and a Kamen Rider in Raising Scorpion form, his counterpart to Realizing Hopper representing his sense of fatherhood and desire to protect Humagears (and a reference to how scorpions actually take care of their young, even though it's the mothers who do that it fits Horobi's theme).
"Sea Scorpion! Zetsumerise!"
"Parent! Forcerise! Fatherise! Raising Scorpion! Protect your family with sting and claw! BrEaK dOwN!"

And lastly, here's a slight revamp of my earlier idea for Progrise key counterparts for all the Zetsumerise keys:
Slashing Mantis
Basking Lizard
Climbing Squirrel
Grappling Squid
Cloaking Bat
Drilling Snail
Springing Frog
Quaking Elephant (a further refinement of Breaking Mammoth that compresses it down to a Maximum-Mighty-X-sized suit)
Blasting Ostrich
Accelerating Rhino (I'm tempted to come up with a "Proto Thouser" form based on this one, but meh, I don't like Thouser)