You punish Bramblestar just enough. Enough to make up for all the times canon and its authors kissed his ass, putting an abusive tyrant on a pedestal over all other characters. It’s both satisfying, compelling, and kinda funny; a fitting end for someone so blatantly yet subtly abusive and corrupt, but somehow also petty and childish in a way no other character is. My favourite staple of Warriors rewrites to show how far their stance is from canon is giving Bramble actual, at times genuinely REALISTIC consequences for his actions. It’s a deserved spit in the face to the idea of hyping up some toxic fragile asshole as a noble hero, like the cognitive dissonance that persists in the books and the authors seem to genuinely believe. Did I mention the lightning is SO funny, it’s like a fire scene in terms of iconicness to me. Deserved. Bramble stans stay loosing, y’all can’t even form an argument.
In terms of the Holy Firestar Lightning Attack, I wanted something quick, "divine looking", and just strange enough that it crossed the border of possible magic. Something I feel fits into Warriors.
I'll never "over punish" a character. Thistleclaw gets a fitting end, ripped open like a bag of doritos by Snowfur, realizing his ideals have actually pushed her away, and that after all this time, Snowfur changed. He dies in complete shock and betrayal.
Tom The Wifebeater gets eaten by One Eye, mind you it was in front of poor Sparrow Fur, but afterwards One Eye uses his dark Sun God magic and consumes Tom's soul as well.
Clear Sky is pancaked on the side of the road by two Twolegs, his body left in the small ditch (respectfully, as they are humans who felt terrible for hitting a kitty and had no way of burying him) before they look out over what is going to be Skyclan's land, realizing how nice it would be to expand Chelford into that area.... Thus setting in motion the events of Skyfall. His spirit is used by Bright Storm to BECOME the Dark Forest barrier.
Gray Wing dies alone and bitter when he pushes away all forms of help, becoming a hateful spirit that perpetuates a curse that's timer has finally hit 0 on, resulting in the death/displacement of all 5 Clan leaders, starting with Riverclan. He also manifests himself through the unlucky offspring of 2 cursed bloodlines (Tigerkin and Applekin) Curlfeather. I'm not giving away his defeat yet though!
For Brambleclaw, I wanted that classic Warrior cats feel, and I think getting smote by holy lightning similar to Mudclaw fits that.
A lightning bolt during plain daylight is already considered a freak event like that has only ever happened once before in Clan history, to the magic user Ṣ̶̫̅n̴̹̯̾̒o̸̜͛ẁ̷͍t̵̰̎ủ̴̲͝f̷̙̳̆t̸̼͐ who got magic BANNED because he used it to ruin Sunningrocks and put generations of bloody wars and pointless fighting in place.
The only cat I've ever gone "too far" with is in an in-universe example of a cat who was punished for her mother's crimes, as her mother had murdered an apprentice. When her apprentice went missing later, she was blamed, and exiled. She also didn't go to Starclan, or the Dark Forest, but rather, somewhere much, much more dangerous.