As for the ponies themselves, I agree with those that said the ponies should be magical and dreamy and less rooted in a human world. No ponies farming pigs,chickens and cows. Farmer ponies would be strictly agricultural and all animals would be able to talk.
I'd imagine the G2 size class to be the best. Anatomywise, the ponies would be more G2 but with thick tails and a sturdier torso. Their heads would be more equine again.
Their eyes would be smaller, not taking over their entire face, but detailed. I would recycle the trend of the diverse eye decal of G4 to give my ponies more character.
Freckles or markings would also appear on my ponies randomly.
Male ponies would not look vastly different to female ponies. I would introduce more male characters but make them look just as bright or pastelly or longmaned than the girls.
No color coding.
Instead I would focus on mould/pose diversity. There would be a healthy mix of colors and not a pink/purple/white dominance.
Pony releases would follow themes. There would be holiday ponies but the holiday would get taylored to fit the imaginary pony world. A thing I liked about FiM - they made the effort to integrate holidays into their universe like Nightmare Night or Heartswarming.
No country exclusives. All people worldwide should have the same assortment. The local stores can vary the number of items they stock but they can't reject types of assortments (remember that this is not a realistic scenario
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High adventure with a dose of slice of life would be ideal for my TV show. Instead of two-parters I would have TV specials once in a while that can be split into episodes.
The show has an epic plot and new pony toys get introduced along the way. For instance, if I want to roll out the Zapper toy, Zapper pony gets introduced. If the pony is not supposed to have a strong toy presence it will appear as a returning/talking backgrounder.
Ponyland would be held up as an ideal commune with threats coming from the outside. Ponies would not be racist towards each other based on their species (something I hate in FiM with the overpowerful unicorns and outcast zebras) or sexist. Note that I eradicated sexual dimorphism, so there is no need to gender jobs or functions. A strong mare can fell trees and a nurturing stallion can be a softspoken teacher
Badguys that are threatening the lives of others will not get reformed, even tho they will be written to be multidimensional and not just evil for the sake of evil.
Focus on symbols/marks would remain in the sense of having a celebration when a pony gains her mark and working the symbolism into the pony's character. But ponies would not choose their jobs based on their marks, the marks would contain a pony's magic and be a representation of what makes a pony unique.
All ponies would have an amount of magic.
No species would be more powerful than the others when it comes to the use of it.
Seaponies, Flutters, Hippogriffs and other equine fantasy creatures would be versions of pony. They would have magic marks like them.