Now we turn back to chapter 136.
The fact is on the day that Tsuna and Reborn go out to see Lanchia and Basil off. Lambo suddenly appears running toward Tsuna. There is a shadow man secretly looking at them. Lambo fires a 10-Years-Bazooka to Reborn. Reborn says "This is not good. I can't move."After that Reborn is hit by the 10-Years-Bazooka, but his his future self doesn't appear. 

There is 3 mysteries in this:
First: The fact is on the day that Tsuna and Reborn go out to see Lanchia and Basil off. There is a shadow man secretly looking at them. The fisrt question is:
1. Who is the shadow man?
We all agree that this is a MAN by the shadow figure of his trousers and his shoe.
We all agree that shadow man should be someone from the FUTURE. He knows about Tsuna and Reborn. Probably Tsuna and Reborn know him too. That's why he cannot appear before them. Or there will be time wrap or confusion.
But it links to another question:
2. How come someone in the future can go back to the past?
Right at that time, no one know! If the technology is more developed in the future, they can made a time machine.
And another one is on that day, no one knows that Tsuna and Reborn go out to see Lanchia and Basil off because they just suddenly decide to go back.
3. Why does the shadow know about this so he could secretly watch them?
If he came from the future and knew that fact about that day. But only Tsuna, Reborn, Basil and Lancia from the future knows where they were in the past.
This theory is uncertainty because anyone could go to look for Tsuna & Reborn but only if he arrived to the past before that. But we can use it as a reference for the next theory.

Second: The fact is when Lambo fires a 10-Years-Bazooka to Reborn. Reborn says, "This is not good. I can't move." and then gets hit. The question is:
4. Why can't Reborn move?
We agree that the shadow man did something to Reborn that made him couldn't move.
But it links to another question:
5. How can the shadow man make Reborn cannot move? 
Right at that time, no one know! Even Reborn doesn't know either (chapter 140)

Third: The fact is, when Reborn gets hit by the 10-Years-Bazooka, his future self doesn't appear.
6. Why doesn't Reborn future self appear? 
If we follow the funtion of the 10-Years-Bazooka, it switches the present-self and the future-self. Even if he's dead in the future, there should be his body laying down somewhere. But in this case, Reborn's future-body didn't appear when he got hit by the the 10-Years-Bazooka.
7. So where does his future body go?

Like Hayato said Reborn doesn't appear when he gets hit by the 10-Years-Bazooka because he does not "exist in the future."

This gave me a strange hint about that "Reborn's future self was already in the past." 
This can explain for question 6 & 7 why his future's body didn't appear.
This also explains for question 3 why he knew where Tsuna and his past self were on that day.
And of course he possibly knew how to restraint himself so this can explain for question 4.

So in the other way, the Future!Reborn is still alive. 
And he’s possibly responsible for sending the Vongola Guardians and Rings to the future.

That’s my theory.

After all, Reborn is the first person who is sent to the future. 
And one more thing he is the MAIN character so he can not be dead, though this is an uncertain theory.

Next in the fourth mystery:
The fact is after Tsuna got himself hit by the bazooka and was sent to the future, he found himself in the coffin with his symbol marked on the coffin (chapter 136).
8. Is the Future!Tsuna dead?
If so, according to the 10-Years-Bazooka ‘s function, his future body would have gone back in the past, in Tsuna’s room and Lambo’s also there.  If he’s dead, would Lambo have to freak out or something? Moreover, Gokudera arrived not long after Tsuna, and he said the rocket flew out of the window (presumably Tsuna's bedroom on the second floor) and hit him (chapter 137). It means somebody in Tsuna’s room fired the bazooka at Gokudera. The questions are:
9. Who fired the bazooka at Gokudera from Tsuna’s room which's having Lambo and Tsuna future’s body in?
10. What happened to the bazooka after Tsuna got hit?