Can 2nd gen GSC games get an expanded pokedex rom?

Hello, i'd like to ask you all why isnt there GSC games, gold silver and crystal who don't have more than the 151/251 mark? The reason i'm asking this is because we have rom hacks of 3rd gen games such as hyper emerald Z and fire sun who have over 800 pokemon in their database but those are 3rd gen template games, why don't we have one for the second generation? Is it because there's no one bothered to make sprites? or is it because there's no program to do it?
I remember sanquii randomizer they did something neat for pokemon Red they had increased from 151 to 251 and they added all 721 pokemons but they couldnt all appear at the same time, it randomized which pokemons out of those 721 would appear and only 251 were there.
Take it as sort of like how sword and shield will be like, let's guess that there will be 905 total pokemons by the end of gen 8 but only 450 will be elligible to catch, it was somewhat similar how sanquii red randomizer worked.
What i want is to know if there's a possibility for a GSC game to have all 809 and if there's any current project which is working on expanding the total number of elligible new sprites and new pokemon one can add to GSC.
( I am well aware there are GSC roms with "new pokemon" but they replace the old ones that's not what i want. )
Sorry if I come of as petty, I don't have knowledge on how to rom hack, I just wanted to make a simple yet technical question :D i'd love to know if there's limitations to the GSC engines to hold more than 253 "max" count on pokedex.

This is not really too feasible due to limitations.
Pokémon Crystal uses a two-digit hexadecimal system, so the max number is 255. Of course, one is used for eggs, and another is null so that the game can recognize an empty slot properly.
If you wanted to go past 253 Pokémon, then, you'd have to tear down and rebuild the indexing system altogether, which would be an overwhelmingly large amount of work (and requires redoing all the game's functions in some way).

I don't know how feasible this is, but Rangi's ToDo page for Polished Crystal mentions an optional plan:

Rewrite Eggs to use the IsEgg bitflag instead of $fe
Rewrite everything to use two-byte IDs, allowing 65,534 Pokémon

Click to expand...

It's more likely that she'll implement regional variants and use the egg slot for one more Pokemon, I guess.

That sounds awesome, i'll follow her up on this then, and thanks to Illyasviel for the technical reply, now i understand what's limiting it.

yes
someone working on Pokemon Brass, formerly Majora Crystal, figured it out

I'm not seeing anything on Majora Crystal's Github to indicate the kind of massive overhaul needed to use a 16-bit value to index data tables relating to Pokemon. There's some Alolan forms, but they're implemented by special-casing particular species and checking a bit in the Pokemon data structure to decide what data to use.

ThomasWinwood said:

I'm not seeing anything on Majora Crystal's Github to indicate the kind of massive overhaul needed to use a 16-bit value to index data tables relating to Pokemon. There's some Alolan forms, but they're implemented by special-casing particular species and checking a bit in the Pokemon data structure to decide what data to use.

Click to expand...

It's this one https://github.com/WasabiRaptor/pokebrass