This sentence doesn't work, I'm sorry to say. You have given no context and why does your question need to be 'formal'? Of course it needs to be polite but good simple English is usually best and easier to write without mistakes. To start reconstructing your sentence, take out the would you suppose and ask a simple question. You should find that the word 'that' is incorrect. Then convert the verb 'is'/'will be' to 'would be' to soften the question. Add 'do you think?' at the end.
In order to know whether any sentence is grammatically correct we need to understand what you're intending it to mean. Please provide more context about what you want to say, and what you mean by "formal".