I'd guess part of the issue with the lack of a third belt in the main toyline of Zi-O was just the production team itself. Based on some recent interviews, Shirakura dislikes villains that can be active, transform and fight from early in the show. He prefers them as background figures rather than like active members of a standard tokusatsu "evil organization" (with rubber suits and such), something he considers taken up by Sentai nowadays.

That's probably why we didn't get an extra belt and Swartz only got a transformation in the final arc itself. It's probably not something solid enough yet that Bandai absolutely enforced on him, and with the Watches selling well there was no need to force it on the show's production.

Bandai decides all this stuff well in advance. A hypothetical Third Driver would have been decided upon very early in the show’s run, before they could know how well Ridewatches would sell.
Shirakura would have literally zero control over things like that, as it’s all be decided months in advance by Bandai.
Besides, not all third quarter belts are used by evil guys. The Eyecon Driver G was used by Takeru, and Chase only became a Rider after he had already become a good guy.

https://pbs.twimg.com/media/EDsxsurU...png&name=small
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Kamen Rider Thouser and his ThousanDriver. Also the Thousand Jacker. He uses two RiseKeys to Henshin and so does his weapon, for finishers.

NVM weapon only uses one. But it's basically the Beat Crosser. You pull the ring, then put in a RiseKey.

so silver, gold and blue color scheme. and it's the 6th rider of the show...
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Why do I want to make super sentai 6th ranger jokes

He looks like Blade King and Jack. He definitely looks like the main villain. He reminds me of the Shadow Kaijin from ToQger. The heroes look very simple, but he looks detailed and complex.

Meteor Storm and Beast Hyper comes to mind. :lolol

Even the initial toyline is designed in conjunction with the show's team even if to a limited extent (see Tsukada being the one that pushed green/black for W's colors) and the later quarters are much more strongly influenced by the series itself - it's why for example you get Cross-Z going from a sword wielder to a knuckle punching Rider in his later forms. The original form was designed before his character was set, but the later ones were made with Banjo in mind.

Also, Ghost's case is the oldest of the trio and clearly didn't set the trend for how they'd handle these. The third belts in both Ex-Aid and Build were just modifications of existing designs, something that easily might be added due to influence of the actual show production. They weren't original core designs for the toyline, unlike the sequence of main Rider upgrades for example. So, it's very likely they wouldn't exist if the producer of those series didn't want them to exist, unlike the actual unique parts.

The team at toei can affect some things - like designs and colours, but they likely can't add (unless Bandai have reason to believe it'll sell) - and definitely not take away, things Bandai wants to sell. Like, they couldn't tell Bandai not to make a belt if Bandai thinks that it might sell. Not to mention that the one time we know Toei did ask Bandai for something - a Final Form for Kaito - all Bandai gave them was Lemon Energy, which didn't get any unique toys for Baron.
The most likely reason for Bandai not doing a third belt in Zi-O would the Evol Driver and/or Buggle Driver Zwei not selling as well as expected.

I really don't think a repaint or minor modification is "taking away" though. Pushing to avoid one of those isn't really the same as pushing against actual unique items.

Baron's situation is kind of different since like you say there that didn't involve Bandai adding any toys, it was just the tv staff using what was already there (with Kaito's popularity being used as justification for the budget for the new helmet prop which was necessary for the form).

If they were actually core of the toyline and Bandai believed there was a market for them they'd get unique designs though. The fact that they were just modifications of existing items heavily points to them being there due to show staff influence rather than an actual Bandai push. It's very telling that both happened under the same producer too.

If anything Grateful probably didn't sell very well, but the Ex-Aid and Build teams wanted that extra belt to make their final bosses more special, which is why we got those repaints. Even Ohmori's first show also had the Banno Driver, although it wasn't part of the main toyline (only getting release at the end of the show) unlike the later ones in Ex-Aid and Build - which just further points to this being mostly producer driven. He already pushed it before, so they integrated it better in the later series when he wanted it again, which is why they were part of the main toyline rather than a show-only story prop that later got merchandising like how the Banno Driver was handled.