Wish you were here!
If you somehow managed to collect all of the game’s 883 Power Moons (not including purchased ones from shops), there’s a small reward at the end…though I promise it’s better than golden Korok poo. With the final moon deposited in the Odyssey, your sail will turn gold to mark your achievement, and a new portrait will appear in the Wedding Chapel on the moon. There’s a portrait there already that lets you redo the final boss fight and escape sequence, but this new portrait will amp up the difficulty on the Bowser fight by making him faster and more powerful than previously.
If you can defeat him and escape the moon as before, the credits will roll once again…but don’t expect anything flashy. You’ll be treated only to a postcard at the end, showing the gold-sailed Odyssey in the Mushroom Kingdom with all the friends you met during your adventure. Then, Mario will be dropped back in Mushroom Kingdom just as he was before.
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It’s not a great reward for that level of completionism, but at the same time, I’d hand it to Nintendo for not gating really incredible content behind something very few people will ever do. If you’re that big of a completionist, you’ll have to be content with the achievement speaking for itself.