I think it's often expressed with the word next, in place of the now, particularly when it's used to form an exasperated exclamation:
Next you will tell me that Rome isn't even in Italy!
I did a Google books search which revealed several contemporary examples. When I performed a similar search using "now" in place of "next", there were several results returned, but most of them seemed to be in an interrogation setting, not the ironic utterances you point to. Here are a small handful of samples:
"Next you will tell me they are sending aeroplanes to the stars, just like to London."
"Don't tell me fibs" — the teacher sounded annoyed — "why next you will tell me that you speak fluent French."
"Now you will tell me what you really are. You will tell me why you have come here with Lieutenant Halfhyde."
"Now you will tell me what is happening, unless you wish to have me tied to my horse's back and carry me screaming aloud all the way to Cornwall!"
I'm not claiming that the word "now" couldn't be used in this way, but I think "next" might be a better choice.
There's also the idiomatic "Now you tell me," which is sometimes said when someone reveals some information too late:
"Oh! I forgot to tell you! We'll need to go pick up Becky, too!"
"Now you tell me! We'll never get there on time."
Your understanding of the word "would" is wrong. When talking about future events the difference between "will" is referring to something that is going to happen in the real world, but "would" is talking about what would happen in a hypothetical situation. So for example, I could say
I would go skiing with you if I had the money.
and that means that if I had more money (an imaginary situation) then I would go with you. Let's take another sentence.
I would go skiing with you if I could get a day off of work.
This means that I have given up trying to get a day off, and I don't think it's possible. In a hypothetical where I do get off work, I would go with you, but in the real world I will not. However, if I say
I will go skiing with you if I can get a day off of work.
that means that I don't know if I can get a day off or not. I'm still trying to convince my boss to let me have a day off, and I don't know if he will let me or not. If he does, I will go with you, but if he doesn't, I will not.
But what if I want to say "It will not happen?" Because the meaning of "it will not happen" is that something is that is unlikely or, worse still, not going to happen, shouldn't it be more accurate to say "it would not happen"?
If I say "It will not happen", it does not mean it is unlikely. It means "In the future, this event is going to not happen." "It would not happen" is only accurate if you're talking about a hypothetical.
Now onto your last sentence:
"I think you will not be able to do it" or "I think you would not be able to do it".
It all depends on the context.
Let's say that I want to run a marathon. I sign up, and start training. The big day comes and I'm really excited. You want to discourage me so you say:
I think you will not be able to finish the race.
New scenario: Let's pretend that I've always wanted to run a marathon, but I am afraid of failing. I want to try, but I want to know that I can finish the race before I make an attempt. I come up to you and ask, "Do you think that I could run a marathon?" You want to discourage me so you say:
I think you would not be able to finish the race.
Best Answer
"I will leave" is correct. This is the simple future. As you are speaking you are still present so when you leave is in the future.
"I would leave" has a number of possible interpretations. The most likely is the condtional "I would leave (if something)" This implies that you are not leaving, because the condition hasn't been met.
("Would" has several other meanings, such as the past tense of "will" in reported speech, and for indicating past habit, but these don't apply to this context.)
I wouldn't use "will" or "would". You are speaking about "right now" so you should use a present tense.
The present continuous can be used to describe events that are happening while the speaker is speaking and will continue into the future. The present continuous is the right choice of tense in this context.