This simple Metroid: Zero Mission hack changes the behavior of red doors so they have to be hit with five missiles or one super missile to open. Not much but good for people who want that old, retro feel.
Metroid - Zero Mission (U).gba
CRC32: 5C61A844
MD5: EBBCE58109988B6DA61EBB06C7A432D5
SHA-1: 5DE8536AFE1F0078EE6FE1089F890E8C7AA0A6E8
SHA-256: FC94F65380B65B870A30B9B04B39CCA1DC63D6E46A4A373D3904ADC0912EBC37