At least personally, its a lot of the shorter, gameplay-focused games that always leave me wanting more, or wanting to further improve, without having some unbeatable new-game++++++++ mode or anything overly RNG based.
A couple games I've 100%ed that still have significant bonus/optional content outside the main plotline include:
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Inscription - Willingly played through the story twice and spent nearly as many hours on the bonus new-game+ mode. Super solid gameplay, that while well explored in the base game, leaves plenty of room to further experiment and perfect your strategy.
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Just Cause 3 - while there is a ton of bonus content, its not overly hidden, and the core gameplay is solid enough that challenges feel fun and rewarding, while travelling around gathering collectables is satisfying in a chill, podcast-listening, but not unengaging way.
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Hotline Miami - after completing the game, I wanted to go back and get a A+ on every level because the gameplay was fun and I felt I still had more room to grow. "The puzzle" wasn't as fun, and I did use a guide, but I was just happy for any reason to play through the game again.
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Wolfenstein the New Order - again, just a solid gameplay loop that made me want to keep playing, with bonus objectives that worked as an objective rather than a chore. Also, unlike later ID shooters, it doesn't have the "beat the whole game without dying" achievement, which just feels too punishing over mistakes that may be minor or downright unfair.
The vehicle challenges were definately the weakest part, but on average, I still enjoyed them. In particular, a lot of the wingsuit courses and land races were fun, and actually took advantage of the game's strongest elements.