To be honest, I would prefer Zi-O to not have a tertiary rider. The story is already confusing as it is. It would be better to focus more on character development than just adding another pretty face.

But that wouldn't be any fun.

A third Rider only will debut by January if there is one. If the story is confusing by then after several months, it's not fault of that additional Rider.

What would you all say if you would make KR Shin's son turned into Zi-O's third rider?

Anyone got more ideas on a tertiary rider for Zi-O?

Maybe a Sundial?

And a driver that actually needs light to transform.

That'd be nice. What would be a good name for the driver? And would it be a belt? How to henshin with it?

Maybe the "Sun Diariver"? (pun on Dial and Driver)

It would require a ridewatch-like trinket but with a pointed end (like a gnomon on a real sundial). When inserted into the belt from the left into the middle, it automatically begins to turn until the point hits 12 O'Clock and is parallel with he sun (of course any other source of light could work, but it's more effective when exposed to proper daylight)

Just like how Ichigo & Nigo's belts used to henshin by using the wind and V3 with fire.

The rider will need a name. And weapons.

When inserting it on the left and then middle, does the driver have the same button on top like Zi-O & Geiz' to spin the belt?

No, I was thinking that it would start to turn as soon as it went in, like an egg timer once it's set.