There should be a version for you as one person also
I feel like "как" instead of "что" here is a valid option, as как is pretty often used in Russian when meaning "what do you think".
It will look less natural in this sentence.
I think "я должен" should be also accepted. It implies a more authoritative style of asking the question.
Could we omit the "you" here? As in
"мне нужно знать, что думаешь"
In a very informal conversational setting you could get away with this, but not in every context.
Is мне надо узнать also a good translation for this sentence or would that rather say "I need to get to know what you think"?
Почему не мне надо знать ваше мнение. Это же одно и тооооже. Why not?