What Are The Limits of Minecraft Modding?

which aspects of the game or things in the game that cannot be changed by mods?

i'm planning for a huge and very complex project that's gonna change a LOT of things in minecraft to the point that it's almost a different game. i just wanna know what the limits of modding are so i can set my plans around them.

Edit: i forgot to say, i'm talking about minecraft java not bedrock.

EDIT #2: thanks for everyone's helpful comments. i now have a much better understanding of this topic. Thanks!

EDIT #3: many of you misunderstood me due to poor communication from my end and me not giving the full picture. so here it is.

I of course will start small and slowly go for bigger and more complex things as i gain more experience. i won't just start working immediately on that HUGE project with like no experience. “Give me six hours to chop down a tree and I will spend the first four sharpening the axe.” - Abraham Lincoln. i do expect the project to take ~5-7 years from today to be finished and one reason is that i'm not going to put like all of my time into it. it'll be more of a side-project. i'm not alone as i will be working on it with a trusted friend which will make it just a little easier. the project is going to be exclusive to ~30 people and i won't get into why that is. and because the project is exclusive (meaning i won't post it to the public) i'm not gonna have to worry about it working on a wide variety of machines nor worry about compatibility as much as public mods. and for versions, i most likely will not update it to newer versions. i will simply pick a version and make it on that version and that's it. i'm a person with iron will so i'm not giving up any time soon. that's why i only asked if it's possible and didn't care how hard or time consuming it is. minecraft and modding in general brings me happiness and seeing myself progress even if slowly is fun.

my goal is to make something in minecraft that have not been ever done before and showcase what the minecraft community is capable of. and making a game myself defeats that purpose.

and lastly, i want to thank everyone for commenting on this post and i respect everyone's opinions on this as it all was very helpful for me. it gave me a much better understanding of it all and opened many possibilities for me that i thought were impossible. Thanks!

EDIT #4: i've already got my answer in the first like 2 comments and the rest just demanded irrelevant context to a simple question and kept talking about that topic instead of the question itself. you don't rlly have to know what i want to do in order to answer that question so idk why ppl asked for more context. anyways this post served it's purpose. i've got my answer. Thanks!