Now that everyone has pretty-much given up on Winds of Winter/Dream of Spring ever being released, something like this recent Robert's Rebellion stage play adaptation just gets ripped to shreds by the fandom as a source for dubiously canonical information, stuff that isn't going to be released any other way. It is like ancient historians dealing with sources like Livy: oh sure, its 50% bullshit, but it is all you are gonna get and maybe if you analyze it just the right way...
The Mad King play does gives us some fun facts to play with - spoilers ahead! I definitely like Aerys being much more "aware" of everything going on around him, straight-up throwing Rhaegar's conspiracies to dethrone him in his face and daring him to do it as a power play move. One of the biggest gaps in the story of Robert's Rebellion as it appears in the books is that it is a bit mysterious why Aerys wasn't overthrown sooner. He was clearly unfit for rule and dangerous to everyone around him. Certainly, it could just be a handwaived backstory detail, it isn't that important after all. Aerys & Rhaegar having very direct drama around said conspiracy though, where Rhaegar just balks at killing his father, is a very logical explanation of how the whole thing essentially dragged its heels until it was too late.
Aerys giving a young Robert a "model" shipwreck of his own parent's death is incredible, that reaches Emperor Palpatine levels of being evil for the sake of the game.
I also like Lyanna being, essentially, a total mystic getting in over her head with her obsession with prophecy and visions. She is one of the least fleshed out characters in canon, and her having something of her own agenda about the whole thing is the right way to do her story. As a bonus, it lets her serve as something of a foil for Rhaegar, a guy wanting to live in the world of dreams but forced to live in the world of politics. She is in a way who he wishes he could be.
Elia serves as the other side of that coin of course, and her being fully aware and actively engaged in the conspiracy and prophecies is, to me, one of the least "spoiler" parts of the new info. She had to be that way, the characterization and actions of the Martells & Rhaegar just don't make sense otherwise. The extent can be debated, I bet the play is hamming it up vis a vis canon - but the direction is clearly correct. And her having her own paramour is probably the best way to make the nature of their marriage clear, and why she would be chill about Lyanna (conceptually).
That paramour being Ashara Dayne is hilarious; if you weren't there you have no idea how much ink has been spilled on fandom theorizing about her character and mysterious suicide, and that was not a popular take. I like this addition, because the previous explanations of "the death of Brandon" etc, all seemed a bit flighty (she did not know him that well in the end) - but losing her entire de facto family once Elia and her children are killed, on top of all the rest, really locks in her despair. Ashara's tragedy is a story I always felt the pathos of, and I think it deserves depth like this.
The one area where I have some big disagreement is Rheagar essentially trying to "sit out the war" at the Tower of Joy, and having to be convinced to return to the conflict? This doesn't make sense. If his side loses the war, he will be hunted down like a dog and murdered! He simply has to know that. I have only read summaries of the play to be clear, maybe there is more to this part of the story - but I think this won't really be canon. Rhaegar is naive but not to this extent.
Also Rhaegar actually beating Robert and then having his killing blow be interrupted by a fucking Dragon Dream is fanfic levels of stupid, absolutely no way that happened lmao. They did that so they could put some incredible dragon puppet dances on stage. No hate at all, for a play that is a great reason. But it didn't happen.
Those are my big takeaways I think! I take the whole thing as probably being "75%" canon - I hope GRRM never says a word about it. Let us wonder, debate, and pick and choose. Maybe we will learn otherwise in Winds...