Well we got Zi-O from the present & Geiz from the future. We need a rider who's from the past.

My theory is that this rider will be someone far from our time, that the Time Jackers forcefully recruit to destroy Zi-O & become the new king. He won't be controlled, more likely forced to cooperate with them if they want to bring him back to his time.

I would prefer Oma Zi-O as a dark counterpart. Not upgrade.

Damien was very clearly talking about the Decade Upgrade Zi-O gets later and not Oma Zi-O, don't know why you felt the need to say that.

As for the main topic, there are honestly multiple motifs the designers could use for the third rider;
-A sundial with a large Necrom-esque horn on the helmet.
-An hourglass would work really well with the rotating driver gimmick.
-Even an old timey pocket-watch with a chain or something for a scarf.

Yeah. Sorry I was thinking final form alternative for Zi-O instead of Oma Zi-O. My bad.

But anyway, yeah a sundial would be nice, Necrom style. Zi-O's got black with pink eyes and Geiz is red with yellow eyes. To balance the colors, I hope for either blue/indigo/purple eyes with the tertiary rider's suit green. Or the opposite. Maybe a grandfather clock? Don't know.

Should tertiary rider get a belt, brace or portable henshin device like the Gashat Gear Dual?

I'd picture the rider's civilian form in Victorian Steampunk fashion. Something like HG Wells' The Time Machine.

I wanna say a brace, mainly because the show is about watches. Although that is assuming Toei takes the low hanging fruit.

Speaking of eye colors, what should the writing style for the tertiary rider be?

Zi-O is Katakana, Geiz is Hiragana, accounting to Wikipedia that just leaves Man'yōgana for the third rider, but there's still the chance they pick something else.

That Man’yogana would be fitting for someone in the past I guess. Either as blue/indigo/purple or green.

And if it’s gonna be a henshin brace, how do you picture it using RideWatches to henshin? Perhaps Showa era riders?

Isn't there a pin for 2033? Seems our third Rider could be that

If we get a "Showa" based Rider for third Rider, I'd guess they'll only use Watches for finishers, rather than armors. Then Zi-O and Geiz use those watches at some point and we get one or two Showa armors.

There are pins for many years, but all those list Zi-O or Geiz's names (aside from Tsukiyomi 2068), so they wouldn't be new Riders. Well, there are a few exceptions - Ancient (0000); Future (0000); Singular (0000); Nothing (0000); Millennium (2000). Still, aside from Millennium and maybe Ancient, none of those sound like Rider names and the year listing seems odd for most of them.

The third Rider likely will have their own Drive, so their name might not be listed in the Zi-O Driver. Using their collectible there could just result in a generic "RIDER" listing.

Really? I would like to see how Showa Rider Armors work though.

And either the 3rd rider's henshin device be a belt, brace, weapon or portable device like Para-DX's?

If this rider would come from the past, would you imagine this rider's civilian fashion to be something of Victorian era with some steampunk?

https://i.pinimg.com/originals/df/90...8dad75baec.jpg
https://d2fzf9bbqh0om5.cloudfront.ne...png?1477008572
http://2.bp.blogspot.com/-b9shoyloaT...cd9b519d4d.jpg

He'd probably wear something from the 70s, or whatever era he's from.

https://mobile.twitter.com/gtrvaried...5824258048?p=v
Zi-o Ryuki armour