>>42352112
>what do
Write what's fun to write. That's the only standard for fanfics you should have.

>>42352782
>an alternate history fic
Your fic can technically adhere to canon if you bump them to alicorns right before the final Discord fight. Perhaps I missed it, but there's nothing in the show that actually confirms they were born as alicorns. All the confirmation is semi-canon stuff like the comics or books.

>>42353087
>It is beyond even our understanding
You can grab onto the "Equestria has never seen before" line, but this basically sinks it. If they were nature alicorns, they would obviously know how they were born and it wouldn't be a complete befuddling mystery to them.

>>42353152
>implying Sombra was defeated before Discord
The Crystal Empire happened "a thousand years ago" per Celestia and we see in the flashback Luna's there, so it happened just prior to NMM and was perhaps an instigating factor in NMM happening. Regarding the Elements, it's possible they were used. The flashback scene's fairly ambiguous as to what actually happens and Sombra was banished to the ice for a thousand years, mirroring what the Elements did with Luna.