You're acting like the storylines that D&D ignored are important.
They might not be central to what the show focused on but they were important. An argument you always see from book fans is that D&D screwed the show the moment they choose to fixate on their favourite characters and ignore everything else, which you can see from how empty the world feels in the later seasons.
- Fake Aegon simply existing in Westeros could tie both Cersei and Dany completely losing their minds. A Targaryen prince that makes his way to the KL supported by stormlanders sick of Lannister's bullshit potentially puts an end to petty succession wars, delegitimatizes Cersei and she decides to blow up the whole city. The reason FAegon is in Westeros is because Tyrion convinced JonCon to invade rather than meet with Dany in Essos, the original plan, because JonCon is the one with greyscale. If Dany arrives in KL and sees a guy that in all likelihood is a Blackfyre on her throne she likely loses her shit and melts everything.
- Lady Stoneheart largely only exists to carry Robb's will: justice for the Red Wedding and Jon's legitimacy. She likely hid it at Greywater Watch. A lot of the undercut storylines revolve around some party trying to restore the Starks through alliance.
> Jon: Robb's plan since he believed his siblings either dead or captured by Lannisters.
> Sansa: Forced marriage to Tyrion. Now a Littlefinger asset under a fake identity in the Vale and potentially marrying Harry the Heir, Sweetrobin's succesor once he kicks the bucket.
> Arya: Littlefinger hands Jeyne Pool to the Boltons to marry Ramsay. Theon frees her and gets them to Stannis who in turn uses Theon for intelligence.
> Rickon: Davos goes to Skagos as part of the Grand Northern Conspiracy to secure Northern support for Stannis.
- Barristan the Bold, one of the most botched characters from book to screen. Last time we saw him he was the one pacting alliances and overseeing a counteroffensive after the cuck slavers were throwing fucking corpses over Yunkai's walls with catapults. Dany is so done with Essos that probably she would be willing to jump Victarion to get the fuck out or get in with Varys' latest scheme. Jorah has just been freed from slavery and is still not over his crush on Dany so he will drag his new mercenary friends and Tyrion to her side because he has nothing else.
- The Greyjoys under Euron are heading into one of the four big battles for WoW, the Battle of Blood against the Redwynes and Hightowers, that looks to establish him as the new big baddie if he really can pull off half the shit he plans. A lot of also barely ellaborated plots in the show converge here: Sam and Sarella, Marwyn, Jaqen and whatever the Hightowers are up to with a side of the great maester conspiracy. Victarion was likely send to fetch Dany because Euron knows what he plans on doing, GRRM has meant to kill him off numerous times but he just doesn't.
- The Mannis has the backing of the Iron Bank, if he gets a Stark he will get the North on his side, Asha might jump to his side to oppose Euron and there's Aurane Waters who swindled Cersei of an entire fleet and likely will make his way to meet him. He practically solved the wildling issue by creating a new house out of the one good Karstark left. If Patchface's prophecy at Hardhome is to be belived we might see the Boltons get fucking wiped off the face of the Earth with the help of the mountain clans and maybe even the Westerlings, a fucking 180 to the show that already made Ramsay way more important than he originally was.
Shireen being burned comes directly from GRRM. D&D discussed this in interviews that it was one of the "three most shocking twists". Stannis burns her and loses his army in response and is abandoned and basically dies alone.
The reveal of that twist is always really, really vague. It is said Shireen will be sacrificied, not that Stannis will do it or at least willingly. Most fans believe that Melissandre will sacrifice Shireen on her own to resurrect Jon since he's still dead in the books. Stannis loves his daughter:
you will avenge my death, and seat my daughter on the Iron Throne, Or die in the attempt.
Shireen will likely be killed by her greyscale and her friend Patchface likely plays a role in it alongside the Ironborn religion of the Drowned God. Patchface's profecies are one of the biggest loses for the show because he literally spells what's going to happen.
Tons of cool shit that show watchers never got to see, yeah no?