Mondo!peSTIcIDes No.36781577
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Broad changes is right. If Sunset became friends with Shining Armor and Twilight...man, stuff could've been way different.
She's already learned friendship at this point, making her the successor and possible element of magic. Twilight would no longer be taken on as a replacement student, and neither would be sent to Ponyville to make friends. Twilight's fate might redirect to being the student of her brother's new girlfriend, Sunset Shimmer, rather than her current tutor.
The Elements of Harmony would be completely different, with Sunset as the core. Though Shining Armor's close relationship might lead to a spot on the team. Maybe this couple would be all there is, like Celestia and Luna covered the whole bunch back in the day. They could probably deal with Nightmare Moon and Discord as they stand, but what would happen with Chrysalis?
Presumably, she'd still want to invade, but will she still mimic Cadence? Will Cadence even have a partner anymore? Does she mimic Sunset, instead? And if so, will she be stopped? Perhaps this all falls on the eventual marriage of Shining Armor and Sunset Shimmer, and Twilight still shows up to crash the party. But what if things happen completely differently, and the victory of the current timeline doesn't occur at all? All of Canterlot lost to the changelings, with the elements of harmony and one of their primary users locked away separately? Bad End.
And if they do succeed there, what happens in the Crystal Empire? It comes back on schedule, perhaps Cadence is sent up there to manage the place. But without Shining Armor, how long can she last? And without a test for Twilight, who will end up acquiring the Crystal Heart? Maybe the Sunset/Shining duo can still save the day by arriving to assist the unrelated princess, but again, maybe that crushed butterfly dooms the Crystal Empire to servitude under Sombra once more.
The rest of the show is even harder to parse, because it revolves around Twilight becoming an alicorn. But if Sunset learned friendship as Celstia's successor while Twilight was still a filly, it's likely she'd have ascended by now, too, so half of those plotlines are erased. Discord might not meet Fluttershy, so he'll still be stoned when his vines strike, though Sunset and Shining might still be able to save the princesses. Losing the elements in the process, they'd still have to discover all the keys on their own. When Tirek escapes, Discord won't be there to stop Tirek, but he also won't be able to help him. Will he get shut down early, or rampage freely? Who can say?