ROM hacking has come a long way since the days of crude sprite edits of Super Mario Bros. Now, modders are achieving massive feats of wizadry, such as cramming an entire game into another one. A Modder going by Trenavix has done just that. Using Super Mario 64 as a base, Trenavix has brought full levels from Banjo-Kazooie into the game. They work and are playable.
As shown in his Super Banjo-Kazooie 64 REDUX demo, Trenavix runs around Spiral Mountain as Mario, and they demonstrate how they have placed enemies, coins, and more around the area. This is not a port of Banjo-Kazooie with Mario in it, but rather more of an “import” that brings the environments of Banjo-Kazooie into Super Mario 64. As it is now, as the items, enemies, and such are all Mario 64.
The demo goes on to show a locked Mumbo’s Mountain, which Mario then collects a Jiggy to open up. He’s then able to go in and explore that level as well, which has Jiggies set up in their original locations, along with things like Goombas and red coins. It seems like there’s a lot of potential when it comes to what can be done here, and Trenavix is merely scratching at the surface.
Source: DSO Gaming