If Twilight Sparkle hadn’t become an alicorn, how would the story of ponies in Equestria progressed? In thinking about this, we must first consider why Twilight became an an alicorn in the first place. Put simply, Twilight became an alicorn after Lauren Faust left the show due to Hasbro increasingly taking creative control over her ponies away from her; Hasbro wanted to sell more toys. Twilight being an alicorn meant that they would have another pony toy they could sell. In addition, with Twilight being a princess, this meant that they could market an entirely new product line based off her now being a princess, thus appealing to the princess fantasy/aesthetic. With Twilight being the main character of the show, they would stand to make a lot of money with the new product line. But what if Hasbro hadn’t got rid of Lauren Faust and what if money wasn’t a motivator at all in the creative direction of the show?
I think one possibility might have been that instead of there being an increasing number of alicorns in Equestria (starting with Princess Cadance — also an alicorn to sell toys), then Twilight Sparkle (which makes little sense, given that the sun (represented by Princess Celestia) and the moon (represented by Princess Luna) perfectly symbolize yin/yang (further supported by the design of the Equestrian flag that appeared in Season 1 episode, “Hearth’s Warming Eve”) …something else might have happened. Instead of Celestia raising the sun, Luna lowering the moon, and Twilight making a pretty friendship cutiemark in the sky and smiling and waving, what if Celestia and Luna simply stopped being alicorns?
This, however, would hardly be considered because the short-term profit for Hasbro would simply be too low. Why would anyone be interested in buying a non-princess unicorn Celestia or Luna when they weren’t even princesses, they would think. Even so, there was an episode in G3 in which everypony became a princess — and it was great and nopony changed form.
Theory: Princess Celestia and Princess Luna needed to be princesses before Twilight Sparkle mastered friendship, becoming the “princess” (in Hasbro terms) because there was not yet a fully-friendshipped horse in Equestria yet. If we separate out the role of sun-raiser and moon-raiser from alicornhood, then you just have two regular horses — one that’s really good at raising and lowering the sun and one that’s really good at raising and lowering the moon. Perhaps their princesshood is unrelated to their roles …maybe they became princesses because there was no friendship and Equestria had need of rulers. (There’s a similar story in the Old Testament of the Bible, about people not wanting to follow God and demanding God give them a king.)
So the theory goes that Celestia and Luna weren’t born alicorns — but rather unicorns, becoming alicorns later on in life, perhaps after everypony got impressed by their extraordinary powers… Thus, when Twilight finally mastered friendship, I think the episode should have resulted in Celestia and Luna losing their wings and thanking Twilight for removing the curse of princesshood from them. Instead, however, Twilight ends up getting sucked into the princess-cult too, with only a passing line from Celestia, telling Twilight that they are her students now, too. …which is great, but far too complicated.
While Celestia and Luna would have been cured of their curse, they would retain their powers, as they were born with them. It might have been even more interesting if they found out in a later episode that their cutiemarks had been forced on them all along, as well — and there could have been an episode of them trying to receive help from the Cutiemark Crusaders in order to find their real ones (for example, with nopony as powerful as Celestia and Luna to raise and lower the sun and moon, and with them doing other things now, then how would there be day/night)? Instead, we get an somewhat obnoxious unrelated S9 vacation episode.
In any case, I think this would have made for a more interesting next several seasons, as well as done a much better job of creating a G4 that would have continued with good episodes indefinitely. This general idea that I’m talking about is not new or unprecedented; it happened in Aladdin and Magic Knight Rayearth, as well — and probably countless other anime and shows but these two come to mind.
In my headcannon, then, I think Celestia and Luna become unicorns and eventually get their cutiemarks. Twilight doesn’t become an alicorn. Eventually, all the ponies learn that they are princesses, too (just not alicorns).