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."
The last phrase of the sentence is fine.
But the comma is not needed.
The word order in the beginning should be different too. If I hear I cannot imagine how he will be disappointed, I would understand that to mean that you do not think he will be disappointed. "How" would refer to "in what manner?"
I would word the beginning like this:
I cannot imagine how disappointed he will be
"How" in this way would refer to the amount of disappointment.
Best Answer
As FumbleFingers mentions in his comment, this structure is fine and not uncommon. In English, we often modify the sentence to match a particular perspective, in this case the perspective of Elena, from the future, looking back over something I did in the past. This in comparison to:
This perspective is of me, looking forward to something I will do, which Elena will later correct.
This can be tricky and can be ambiguous without good time markers. Although you provide this information in your question, in your actual example it's not clear if you will miss something in the future (from when you are speaking) or you've already missed something in the past. In casual conversation, this information might not be important or you might have already provided context in the previous sentence, so it's not a big deal. Just something to think about.
Consider these two examples:
Because I'm indirectly quoting Elena, it's more apparent that, if I say "miss". it means I will do the task in the future, and "missed" means I've already done the task.
Of course, this assumes that everyone chooses their words carefully instead of talking as they are thinking. Native speakers often say sentences with confusing or ambiguous verb tenses, and you have to parse the actual meaning from context.