When I mastered Super Mario Galaxy with 120 stars, I immediately replayed it with Luigi and mastered it all over again. Suffice to say, I obtained 121 stars with both characters in a week of the release date.

Also, I'll usually replay a game with a new game+ feature if I like it enough. I used to immediately replay finished games a lot more years ago. Games like Brave Fencer Musashi, and Final Fantasy titles, etc.