Colin warning:
Ok. I went back and watched the last 10 minutes of the episode again and I think I have noticed a few things that make it more clear what happened in the last sequence with Hodor
- Bran didn't voluntarily go back to past Winterfell. The TER took him there because he had to show him something that would prepare him for what was to come
- Bran is already Greenseering mode when the Night King attacks. However, he's watching his Grandfather, Dad and Uncle and not really paying attention to young Hodor
- It's only when Meera frantically shakes Bran and says "Warg into Hodor now" that he responds and turns his attention young Hodor
- The TER also encourages him to listen to Meera and finally Bran wargs into Present Hodor. At this point in time Young Hodor hasn't responded in any way to what is going on with Bran or his future self
- Present Hodor gets Bran up, Summer dies, TER dies after telling Bran to Leave him, Leaf dies and Hodor and Meera make it to the door.
- Meera calls Hodor's name a few times at the door and then finally they get outside and frantically tells present Hodor. "Hold the Door"
- Back in the flashback. Bran turns and sees that young Hodor is already looking in his direction and seemingly responding to Meera's voice (which is the only voice we hear now).
- Finally we see Young Hodor eyes roll up and he falls out and starts repeatedly saying "Hold the Door"
I think what the sequence tells us is that although Bran's greenseer ability provided the "conduit" for Wylis's transformation into Hodor, Bran didn't actively do anything to trigger it. He didn't warg into young Hodor and young Hodor was not immediately affected when Bran warged into present day Hodor.
On the other hand Meera seems to have shown an ability which no one else has demonstrated up til now. First, she was able to reach through to Bran in his trance state and impact him. Secondly, it was Meera calling out of Hodor name and commanding him to "hold the door" which cause Young Hodor to react and finally sieze up. She clearly can't see the past the way TER and Bran can and she hasn't shown any Warg ability to date, but it was definitely her voice which got young Hodor's attention in the flashabck and triggered the Hodor state.
So to me we now know that Bran is limited in what he can do while in vision mode. People can sorta hear a voice, but can't understand him. I think that was TER's final lesson for Bran. Meera on the other hand seems to come through loud and clear and with enough impact to cause a physical reaction.
So here is my new theory: B(ran) + M(eera) = Time Master!
The only supporting evidence for my theory is that Meera survives the episode when literally everyone else on Team Bran dies. That tells me she probably has a larger tole to play and it may also give further support to the R + L = J(on) & M(eera) theory which I didn't really give any thought too until now.
ON THE OTHER HAND -
All this could be true, but only for the special circumstances that were present at the time. Specifically, Hodor was present in both the green vision flashback and the present which may have provided the extra mystical mumbo jumbo for Meera to make a direct connection through Jon. In which case my theory would only be meaningful for Bran and Meera when the same circumstances are present.