Okay… I just noticed some things… And I have come up with a crazy theory. But first, let's look at some canon.
1: Twilight became an Alicorn thanks to achieving the highest level of understanding of Magic and Friendship. Which is both her Element of Harmony, and her Special Talent. She also performs something with the full power of this knowledge to do so.
2: Cadence, as noted in the Crystal Heart Spell (Book), being a former Pegasus, became an Alicorn by a near total understanding of Love. She also, from what I've heard, performed some demonstration of that knowledge to do so.
That there already shows a working theory I stole from someone else; that going beyond mere mastery of one's talent grants Alicornhood under certain circumstances unique to the pony.
(Note: It would be rude to say that it's not possible for your everyday work pony talents, cause Greek mythology had Hephaestus: God of Blacksmiths.)
But working off of this, there is an open question that needs to be addressed.
What about some other ponies we know? How would they reach Alicorn-hood?
Could Dash reach Alicornhood by going beyond mastery of knowledge of what Loyalty means?
Could Applejack reach Alicornhood by going beyond mastery of knowledge of what Honesty means?
Could Rarity reach Alicornhood by going beyond mastery of knowledge of what Generosity means?
Could Pinkie Pie reach Alicornhood by going beyond mastery of knowledge of what Laughter making others happy means?
Could Fluttershy reach Alicornhood by going beyond mastery of knowledge of what Kindness means?
(…Wait, that last one sounds like it's not far off. Fluttershy next Alicorn Princess?)
I'm going to assume yes, seeing as our only evidence we have points that way.
Of course, that still leaves us with a large question of what each pony's "Great work" to have them exemplify their Element would be. Which I'm assuming is a large part of the process considering both known ascended Alicorns have done one.
(Which in mythological terms is reminiscent one of the parameters of what one needs to do to fulfill "the Ultimate Alchemy". Which from my limited knowledge sounds remarkably like the process for becoming an Alicorn.)
Now, with that taken care of saying, I get the distinct feeling someone should make a Fan-fic about an Earth Pony Blacksmith becoming an Alicorn Princess by going above and beyond mastery of the art of the forge… Crafting some legendary weapon or something in the process as well to act as the catalyst.
Cautious Edit: Do not confuse me raising the possibilities about Alicorn ascension with me endorsing it happening again, saying it will definitely happen, or that it will happen at all. I'm just explaining the how and why of what makes it happen. Any judgement on those other issues is something I withhold until we have something to work with.