What if Trump attends Chinese Military Parade on September?

i can't imagine him willingly going there to getting humillated by the PLA parade.

Nothing. Nothing substantial would change. It would be mildly amusing to see the reaction of the media and anti-China hawks in the US and Europe but that's about it. We'd have a few weeks of entertaining content and then the news cycle would move on.

Liberals would liken him to an "evil Chinese dictator" stereotype like they try to pretend him and Putin are "evil Stalin" stereotypes. Probably see memes of Trump as Mao, and a revival of anti-CPC discourse, but applied to Trump as "ruining America" or "selling us how to foreign powers" or "wanting to be authoritarian".

In all the ways that matter, nothing significant. He's a duplicitous bastard who can't be trusted, so he may talk nice about Xi for a week before going "he's not giving us Taiwan even though I asked nicely, what a tyrant" and going right back to anti-China warhawking. China certainly isn't going to do anything for him just because he showed up to a parade. No other country is going to change their views because of it. US rivals know the US is two-faced and vile, and Trump is the personification of it. US allies hate him for saying the quiet part out loud but jump when the US says "frog" anyway.