I would personally try to get a grasp on official material before making original content.
>Get fluent in Japanese
>Get good in various computer science subjects, programming, ASM, math, and ROM hacking across different platforms
>Turn Red and Green back into ASM and learn what every last thing does as well as all the differences between the two
>Remove all unused material as it just takes up space
>Reorganize the entire ROM (Pokémon index numbers now correspond to their Pokédex number, redo all pointers)
>Redo programming for things that don't work correctly or bring about glitches
>Recompile as GB games for what seems like an identical experience, except far more stable and everything works correctly
>Port all this to Java or C# as its own program to avoid Game Boy boundaries
>Make options for Red Mode and Green Mode that switches all the differing elements so the player can catch version-exclusives and see other Pokédex entries just by changing the setting (it would probably require you to save and do a soft reset)
>Add in Blue Mode with its slightly more extensive changes
>Add in the scripts and text tiles for every single officially supported language
>Add a new in-game area where these settings (along with others like SGB colors/borders, GBC palettes, game speed alteration from Stadium, etc.) can be altered at will
>This new area would be west of Pallet Town and would probably only be reachable from teleporting from the Options menu, though it would be the first menu that appears when loading up the game for the first time so the player can adjust their settings at any time
>It'll be a building resembling a giant CRT (hence the SGB/Stadium features) calling itself the "Tube Region Channel" (チューブ地方) (it's a play on the Chūbu region (中部地方/ちゅうぶちほう) since that's where it would correspond to in real life) or "TRC" for short.