Imagine New Super Mario Bros. but made in the Super Mario Bros. 3 engine and conceptualized as a celebration of the entire Super Mario series, including the some of the oft-maligned entries like Super Mario Bros. 2 (USA) and Super Paper Mario. It's one of the most impressive ROM hacks I've ever played, and I mean that sincerely.

I'm only putting it down because I tried to play it on an Analogue Pocket (an original hardware facsimile), and boy does the new stuff in this game bring that device to its knees sometimes. Absolutely worth checking out if you haven't already, but consider doing so on a software emulator.