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When I am joking about something told me, which is not correct, in Italian I often reply with Ora mi dirai che […] (literally, "now you will tell me that […]").
Is it correct to use now in a sentence using the future tense?
A: Roma is not the capital of Italy.
B: Now you will tell me Roma is not even in Italy.
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."
That is certainly the sort of reply a native English speaker might give. Will + main verb, however, is not a future tense. In this example, will is used to make a prediction, even if it is not an entirely serious one.
I haven't heard this usage, and I wouldn't try it because of the reason you say.
On the other hand, substitute the word 'next' for 'now' and you've got an expression well known in English. To paraphrase your example:
A: Rome is not the capital of Italy.
B: Next you'll be telling me Rome is not even in Italy.
Edit:
On further thought, a version that I have heard used is:
And I suppose (or expect) you're now going to tell me that Rome isn't even in Italy.
