Concur it's the plane shootdown.
If I had to bet, I'd say it's video confirming the intel on the missle shoot, and possibly some other intel to back it up.
Since it was a Ukrainian national aircraft, it wouldn't surprise me that POTUS is getting ready to pull another phone call stunt by DECLAS of that (or at least a majority of it).
Just like with the call to Zelensky, though, he wants permission from the Ukrainians before doing so, since it is a potentially sensitive topic for them due to the heavy Ukrainian loss of life.
Just a WAG, but might as well go all the way and say I'd also bet Zelensky says, "Let us publish." Dealing with the corruption in the Ukraine is a serious matter and Zelensky's decision would establish him as a serious, committed ally to that cause. It would be an independent action based solely on their determination and would seriously undermine the whole abuse of power narrative in the impeachment hoax.