[Fish Sandwich]

Honestly, I don't think it has that much of an effect on the way premiere episodes play out either way. A show like Build still has to exposit details to the audience about everything, and a show like Gaim can pretty effortlessly work through that kind of thing without making the pace drag anyway. Actually, Kouta figuring out how the Musou Saber works has far more entertainment value than Build blandly smacking around some random monster at night in my opinion. In contrast, the opening scene of Wizard is infinitely cooler than, say, Eiji fumbling around when he first becomes OOO.

The point is, it's really all up to how the show presents itself. I think what the decision to start with the main Rider already established actually affects is a lot more long-term than that, and once again, it's all up to the kind of story being told. Starting with the origin for shows like Gaim and Ghost is crucial because they're about watching a young hero gradually grow into the role. It just wouldn't make sense to do anything else. A coming of age story like that simply demands the viewer be totally on the same page as the protagonist, so they can start relating to them right away. Given that Kamen Rider is a show aimed at children, it shouldn't be surprising why this is the one they usually go with. Kids tend to be doing a lot of growing up!

Conversely, doing the opposite is best suited to a show where you want to present the hero as having some layers to gradually unwrap, or to have them be more of an ideal other characters strive towards. Build is a great example of both of these angles, because early on, you're largely seeing Sento through Ryuuga's eyes, as this amazing, righteous guy he wants to live up to, building up that image of him to make that arc click into place before diving directly into the more complex, flawed character Sento actually is. Haruto is also one of my favorite Rider protagonists in part because the show basically never runs out of new bits of backstory for him, and every time something about his past is revealed, it puts his character and personality in a new light to some extent. There's this cool fluidity to that, and while I'm sure a lot of you are snarking "Haruto had a personality?" right now, the point I'm trying to make here is that there's no real easy answer to this question for me. I'd lean towards "already a Rider" (I REALLY love Wizard's first episode), but I also think starting from the blank slate with a hero that doesn't know what's going on has a lot of inherent merit to it, and there are just as many of those premieres I love too (Drive and Kuuga stand out off the top of my head).

I also don't think it directly matters to how awkwardly the merch is worked in. Wizard and Build may have both already been Riders, but they were still both getting new gear in every episode, with all that entails. Haruto testing out a new Ring is hardly very different from Eiji testing a new Medal, and since Eiji got most of those by ripping them out of the enemies he was fighting, "become a Rider" show OOO actually tended to take up less of the episode's attention with that than Wizard, which had to fit in Wajima walking up to Haruto to hand him a new toy in one scene or another. The only reason it wasn't awkward in Build is because the show completely gave up on having Sento actually do any experimenting to find Best Matches almost immediately in favor of using Ryuuga as a huge narrative shortcut being played for laughs. One last time, it's all in how the show handles itself.