To be honest, if the rider ends up being completely unrelated to Decade and doesn't even look like him, that would be a huge disappointment.

It does not interfere. Gotchard may be a rookie as a dealer. Like in DnD, when no one wants to be the master and, as a result, he turns out to be the biggest loser.

A DM needs to have a lot of confidence in their memory of the world's rules and their imagination, to keep driving the story and characters forward however the players act.

Yes, but the option that "no one wants to be a DM" is also quite realistic. And then someone new joins you and you find out that you have been playing wrong for a lot of time.
Plus, going back to Gotchard, the options are:
1) He has an eidetic memory, so he can be a beginner and quickly learn everything he needs at the same time.
2) It is just because of his inexperience that he messed up something and a harmless card game will need Kamen Riders;
3) A mixture of the first and second. I went to school with an eidetic guy who managed to get bad grades because he was too lazy to even look at his textbook. With great power comes great laziness.

For sure. Just saying, it's a tough job, so I respect those who take it willingly and competently.

I'm intrigued by your third option. There's a good moral, that even the most talented person in the world needs the motivation to succeed. A super talented person can even occasionally be humbled by such motivated people, like Ace learning the value from Michinaga in Geats Episode 9. Also reminds me of how Abaranger's Doctor Nakadai became bored with living due to how easy his life was, or how Shinnosuke's engine became stalled after his guilt over the Global Freeze.

Just a random theory, and idk if it has been mentioned or not, but what if the “legend rider” in question is not decade but… get this… diend? Maybe diend is our third rider. Could it be?

This aligns with the rumor that each rider will have a unique transformation device. But then let's go further: what if it's Kivala?

Okay so here we got leaks of some toys, the Gotchard Driver & what appears to be a new Raise Buckle prototype (possibly a Gotchard Buckle?)

You can also see the belt strap piece in the picture with the gotchard buckle

Love that you can put a card in the buckle???? Very fun lol

Hopefully we see some official news in the next two weeks, would be inline with the last few years