I checked both the FAQ on this site and the online Japanese guide, and both said that Rivals 350-352 appear in Aso on the Friday of the 4th Week in clear weather (after defeating the last story rival, #324). I've checked for 3 last-fridays in a row and they've never appeared.
In case there are any other requirements for them to show up, I have all 40 prizes, 1370 fans, finished every advance and master race (including Kaido Champion), and defeated all other rivals besides #0 (who you MUST have defeated 350-352 to face). Did anyone ever find out the real requirements, or is there some kind of bug?
I believe that you need more fans, probably around 1500-2000. However, I do remember a specific thing that I had to do to get them to appear. Have you tried going to Aso during the day?
It is highly possible that it's a bug. I, for example, had trouble getting #239 [Legendary Schneider] to appear. However, after much persistence, he finally showed up without me doing anything else in between.
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Thanks for the quick reply! I've tried Aso during the day, but no luck. I'm going to try and get more fans and defeat some more Masters in the process.
I also had trouble with Legendary Schneider, as well as Ootsuka Demon (both were the final rivals of their respective short courses). I was making some corrections to the Rival Guide as I was playing and I wrote that Legendary Schneider only shows up on fridays, and then confirmed it with the Japanese guide page ( http://lod.zero-yen.com/kb3/ ).
OK I checked around the internet and a Japanese board posted this: after defeating the emotional king (blackout) friday of week 4, a clear night in Aso, all 40 prizes, and 2100 fans. MAYBE you have to beat Ground Zero as well (whose requirement is probably 1500-1800 fans, at least he didn't show up before then for me).
I got 2100 fans and they finally showed up. I'll bet fans account for a lot more races than the game lets on. Now I can finally rest easy knowing I've completed the game hahaha.