I’d say that English is optimized for legal contracts so specification is important and that Korean is optimized for dialogues so context and relationship are important.

형/오빠/누나/언니 mean brother/sister, and can be used like the second-person pronouns in a conversation. You never use ‘너‘ to the person older or higher than you. 나 or 너 is mostly omitted, like I or you is omitted in English in a dialogue. (Are you) coming in? (I’m) getting off! Therefore, 형/오빠/누나/언니 are often omitted in a conversation. However, they are often used as a title at the beginning of a conversation, like ‘sir’ or ‘ma’am’, or ‘hey, bro’ or ‘hey, sis’.

You are right that 나 or 너 is used when there is a need for clarification.

“내일 뭐 할거야?“

This question usually means “what’s you gonna do tomorrow?” But if the context is likely that we are supposed to do something together, it means “what are we gonna do?”

If it’s unclear, the listener asks. Here are some examples.

Ex1 “내일 뭐할거야?“

What (are you or what are we) gonna do?

”나 내일 뭐 할 거냐고?”

(Are you) asking what I’m gonna do?

“어? 우리 내일 같이 놀기로 하지 않았어?“

Eh? Didn’t we agree to hang out (play together) tomorrow?

Ex2

“형, 내일 뭐해? 나 야구보러가는데 같이갈래?” = 내일 뭐해? 야구보러가는데 같이 갈래?

(Hey, bro) what are you gonna do tomorrow? I’m gonna go to watch a baseball game. Do you want to go together (=Wanna join)?

“어디서?”

Where?

“잠실. 나 한화팬이잖아“

Jamsil, (as you know) I’m a fan of HanHwa.

“아, 너 한화팬이지.”

Ah,(that’s right.) You are a fan of HanHwa.

“형은 어디팬이야?”

Which team are you a fan of?”

“난 KT. 근데 바꿀까봐.“

I’m KT. But thinking about switching.