I wish the Ark would be the final villain and possibly rider. The Ultron/SkyNet of Zero-One.

Wielding a Zero-One based Driver using Zetsumerise Keys (Ikazuchi only used Dodo on the ForceRiser)

There is no way that any Rider appearing that late would get a bunch of forms. Even Evol's four just used the same suit with different heads and shoulders (and, for Black Hole, chest and coat too).

Besides, the Ark was revived using Progrise Keys, so it'd probably just use an overpowered normal Progrise Key, like Assault Wolf.

I see more it being Zea than Ark, because it looks like the kind of twist KR would pull ("Im the one pulling the strings from the start and led you here zero-one").
Or maybe Zea and Ark are one and the same somehow. so it would be valid I guess.

Also I don't know if it's done on purpose or not, but when Jin "dies" in the last episode, and says "the will of Ark blablabla" he is looking up in the sky.

Boooo. They didnt change the henshin phrase.

I'm getting a ooos vibe from zero one with the changing possession of keys between riders

Now that you mention it, when Jin stole the Flying Falcon key from Aruto I was thinking Abaranger when Mikoto hijacked Stegoslidon to form Killer-Oh.

Items shifting possessions is always good, but really they’ve all managed to keep their hands on their full arsenal except for Flying Falcon. It did make me wonder though why Zea can’t just make another one...

Also when do you think they'll make another Assault Grip, just so Fuwa doesn't have to act like a stingy kid not wanting to share his toys.

Your wish comes true in today's episode. :D