We know, there is no Super Mario Bros. 5, but now we can get a taste of what it might have been like thanks to Metroid Mike 64. He has been working on this project since 2015 and has now finished it in Super Mario Maker 2 "trying to create a classic Mario game that plays as if Nintendo created it themselves".
It is basically a full Super Mario adventure in the same vein as Super Mario World, and you can check out what it looks like on YouTube. If you want to play it yourself, the Maker ID is 0G9-XN4-FNF.
<social>https://twitter.com/MetroidMike64/status/1574064194747568131</social>