Before they made Doom, the team at id Software tried their hand at bringing Super Mario to PC. Back in 1990, when the studio was still known as IFD, the developers created a demo for a PC port of Super Mario Bros. 3, which they then sent to Nintendo — but it was ultimately rejected as Nintendo focused on making games for its own hardware. Today, Doom co-creator John Romero shared a lengthy video showing off what could have been for the port.
While it's missing some of the key components — there's no music, the sound effects aren't very Mario, and many of the sprites are changed — the SMB3 port looks remarkably close to its NES counterpart, considering it was made by an entirely different team. There are a few big changes, however. Most notably the second level has been changed completely, so that the blocks spell "Like it?" at the very beginning. Romero shared the video to help commemorate the 25th anniversary of Commander Keen, the platforming game that id went on to make after the Nintendo idea fell flat.
While id has gone on to make very different kinds of games, helping pioneer the now-dominant first-person shooter genre, it seems that the team hasn't quite let go of its Mario love — today former id creative director Tom Hall released a Commander Keen-inspired stage for the Wii U's Super Mario Maker.