Izzy has the “hypernatural ability” to see a pony´s Luminescence.
This is NOT “magic” (Arcana), but rather a psychic ability coming from her mind/soul [like how Angel acquired the skill/capacity to talk & fly alongside Fluttershy´s body & pegasus_magic, but still got “dominated” by THE STARE coming from the eyes of an otherwise “normal” male bunny].
-> Izzy would NOT trust somepony like Starlight Glimmer in the first place. She would panic & run away after checking out any other pony´s sparks (dull, mate, lacking of any true “brightness”), something equivalent for her to a braindead living corpse, moving/running around slowly like a zombie [worse even that the unicorns in Briddlewood, who at least still have their 1/2 cutiemarks].

I think Izzy had confidence on her own capacity to judge & choose who will be the “Brightest Spark” to befriend (from a large group of earth ponies she expected to meet at Maretime Bay); so regardless of how strong his/her pestilence & lack of inteligence may be, at least it would be somepony new to talk, who could bring her some honest, sincere happiness.

@Argamis
This adds a whole nother level of tragic to Izzy. Izzy was so lonely and desperate for a friend that she was willing to travel all the way to an Earth Pony town, despite believing that Earth Ponies are smelly idiots. She’s the kind of pony that would’ve eagerly joined Starlight’s cult and gave up her Cutie Mark, if Starlight had pretended to be her friend.

@Argamis  
I think it would go a different way. Something Hitch said to Sunny: “Prove it.” I think after getting out of her funk she would actually listen and head out to visit a different city on her own. I think she would at least want to know for sure whether she’d been wrong or right.

@Argamis  
That’s dark, man… All end up killed!
 
I would prefer to think that Izzy would be a simple outcast living away not unlike Zecora, Sunny forced into a more monotonous life to better fit in and Zipp abdicating for Pipp and turn into an adventurer of sorts. She maybe even stumble across Maretime Bay’s lighthouse and be the spark for the journey instead.

@Wayneponeth  

 
. Sunny would likely end up permanently imprisoned by either Phyllis or Sprout (or accidentally killing herself).
 
. Izzy would end up “Jinxing” some unicorn (by mistake or as part of a game); then when that unicorn dies from some accident she will be punished like a witch [her babling about “Luminescences” & auras only making her case worse].  
Alternatively: Izzy will die from loneliness (either literally commiting suicide or losing her mind)
 
. “Queen Zephyrina, The Brave & Smart” ruling is brief. She dies on a “parkour accident” [or is actually defenestrated by one of her sister´s “TRUE Fans” (obsessive paranoids) who want Pip Petals as THEIR Queen].
 
The long term consequences of the lack of magic in the world is either: the extinction of all remaining sapient creatures (from those tiny armadillos to Spikezilla, The Last Dragon), or the contamination & corruption of the atmosphere by the growth & inevitable wars between 2 of the pony races (because one of them will get extinct first), posibly ending ALL life.

Technically a piece of paper saved Equestria. Sunny would’ve never thought of going out to get magic back if Izzy didn’t come and Izzy would’ve never come to Maretime Bay if she didn’t see that drawing. The three tribes would never even dare to interact with each other if Sunny didn’t try to get magic back. Sunny’s drawing saved Equestria.

@EquusStorm  

 
The scene were Sunny & Izzy try to hide & cross through the main street while being bombarded by green GAK launched at them is super funny from the point of view of Izzy [she must think is something like “laser tag” for adults].
 
In hindsight, Izzy must be comparing all of this otherwise “strange behavior” of the earth ponies with the quirks of her own comunity (the SUPER_supersticious unicorns performing very silly “rituals” on the spot to cancel_out “bad luck” jinxes); specially considering that she is like an “atheist” for that “religion” (she never gets self_triggered by speaking out loud the “forbidden” words); she simply accepts this idiosyncrasies as they come.  
-> This is how she inmediatly reacted to the “Pip Pip Hurray!” chant in sync, while Sunny was just perplexed.

Oh gawd that theory makes so much sense, I love it. With the just thinking the town was playing games like hide and seek, and complete lack of fear whatsoever of their hostility and attacking, that explains so much.
 
Also yes this scene brings me to tears every single time, just watch it on repeat and completely sit there breaking down lol

@Background Pony #6680  

 
Headcanon Accepted.
 
Izzy assumed every earth pony would behave like a foal, regardless of their actual age.  
This explain the way (“big” baby talk) she used to comunicate with Sunny and other incidental ponies around, including when she said goodbye as she was runing with Sunny to the lighthouse, or her attempts on “diplomacy” with Hitch & Sprout.
 
The way she quickly answered with one word the first of Sunny´s “Questions for a unicorn” & then attempted to “distract” her by bouncing & opening the can (instead of simply telling her the truth: that she could not make the can float); is a similar strategy adults would use to avoid making a child sad & dissapointed.  
-> The first time Izzy talked with Sunny like she was an “equal” to her (about something highly spiritual & abstract as “Luminescence”) was after days of galloping on the countryside [having experienced enough adventures together to determine that Sunny could even be smarter than herself].
 
Pegasi also think earth ponies are dumb. Zipp complimented Hitch on his “sudden realization” that the daughter of a Queen is a Princess like it was an achievement [later on the campfire scene she feels pity of his failed attempts at igniting a fire].  
-> She likely took Sunny for a “genius” of her kind (someone either “selected” as the BEST for this quest, or rather the only one who could have thought of it on the first place).

@Background Pony #6680  
I don’t know. Think about it: while Izzy is friendly she was not yet at Sunny’s level of trusting/pony unity idealism as she was afraid of pegasus and wanted to avoid them. Yet Izzy walked straight into an earth pony town alone. Why the difference?
 
I think it was both Sunny’s letter and the idea that earth ponies are more “harmless”–which seemed to be spread throughout the other towns–that made Izzy take a chance to visit Maretime Bay.

@Softy  
IT MAKES SO MUCH SENSE

@Softy  
bruh it still looks better than anything I could draw by hand  

@Background Pony #6680  
Holy hell, that’s hilarious.
 
Izzy was taught that earth ponies “weren’t the brightest crystal in the forest”. Maybe she thought this was drawn by a full-grown adult.
 
Reminds me of Fry’s drawing from Futurama.

Never say the thoughts of another stop mattering when today becomes yesteryear.
 
Feelings are forever ponies, be kind to one another. <3

What if Izzy came to Maretime Bay thinking that message was written by an adult?  
Because she clearly treated earth ponies like babies the first time.

 
ngl I choked up a little