New Super Mario Bros.'s implementation of Star Coins was a good one, it was the only entry in the series where you can spend them on cosmetics (that being your bottom screen wallpaper) and I really wish Mario games had more cosmetic options. Mario's costumes in Super Mario Odyssey was very great and gave a large incentive to collect the coins.
I would love Mario Kart's part unlocking if you could get the car you want and if there are more customization options than just body, tire, and glider. Would love to spend coins on, more cosmetics as well as an expanded customizable car, but of course, don't copy CTR and monetize it and do a terrible grind because that's a fucking assholish thing to do.
Mario Party DS has the best collectibles and gave you incentive to play through the game, especially the boss trophy ones. Any game with amusing descriptions for their collectibles makes them rewarding to unlock them. This also includes Wario: Master of Disguise.
