I always liked the idea of pairing the Humane 7 with an element. Since that will be crucial for a story I'm writing, I need to assign an element based on their personality and the "pony power" they have. Turns out it was a lot more difficult than I thought. I got the obvious ones down.

Applejack

Power: Super strength

Element: Earth

Rainbow Dash

Power: Super speed

Element: Air

Sunset Shimmer

Power: Mindreading by contact

Element: Fire

Twilight Sparkle

Power: Telekinesis

Element: Arcane/Magic

I was thinking Fluttershy could be nature, but since nature is everything, I'm not sure if I could choose that. Then there's the water element, which I thought it could maybe be Rarity because personality, but it's a little weird since she has 'diamond powers' and her geode is a picture of diamonds. And then there's Pinkie, and I have no idea what her element could be at all. Suggestions are welcome, and I don't mind changing the ones I have already if there is something that fits them better.

Fluttershy: Water

Rarity: Metal

7049668
Maybe? Still gotta find one for Pinkie, tho.

Pinkie: Space

7049739
On second thought, change Fluttershy to Wood, and Pinkie Pie should be Water. Fixed it!

7049653

I'd pair Applejack with Wood personally. She's heavily associated with trees, and indeed trees are about the strongest things you can find if you take their creeping pace into account. A tree can shatter concrete like it's not even there, given a few years to do it, with patience and strength.

Sounds like Applejack to me...

7049653
Funny you should ask, because I've been thinking about this for a group of expy characters I made for one of my original stories.

I plumped for the five elements in Chinese philosophy:
Earth
Fire
Metal
Wood
Water

Under the Chinese system, air is not element because air is qi and therefore a universal life force.

I only have six characters to deal with, so this system might not be suitable for what you're thinking about doing, but if I were to try to slot the Humane Seven to the elements, they'd fit as follows:

Fluttershy - water (for the emotions of fear and gentleness)
Applejack - earth (for the agreeable and honesty)
Rainbow Dash - fire (for the passion and intensity, plus fire is associated with feathered animals)
Pinkie Pie - Wood (for the mental qualities of spontaneity and curiosity).
Twilight - metal (for the mental qualities of intuition, rationality, mind)
Rarity under this system is problematic because she straddles water and wood, plus Sunset is also spread out amongst the elements too.

7049653
Under the traditional elements of nature in fantasy, there are:
Fire
Water
Earth
Air
Electricity
Light
Dark
Mind

Assigning them to what I think matches both the character and their ‘Harmony’ powers, I’d go with

Mind - Twilight
Psychokinesis - Most of her powers come from moving things with thought, but she can also make portals, as evidenced by Midnight Sparkle. With enough knowledge of how any system works, one can literally do anything with this power. This would be directly imposing one’s desires on reality by bending/tearing space and manipulating matter, hence, Mind.

Earth - Applejack (duh.)
Super strength/durability - Applejack’s honesty is a preservation of objective Truth. What is stronger, more immutable, and enduring than absolute Truth? It is the bedrock on which a stable reality is built.

Light - Rarity
Illusions/Energy constructs - Rarity’s obsession with the visual arts gives her a penchant for creating things that manipulate and guide one’s perceptions, but with the ability to manipulate energy, she can also construct anything she can visualize from light and harden it into physicality.

Electricity - Sunset
Emotion sense & manipulation/Electrokinesis - Just about all carbon-based lifeforms and their brains work on electricity and the brain gives off significant electromagnetic radiation. Having an intuitive understanding of what certain patterns and signals mean would allow Sunset to understand how someone feels, but also to induce certain feelings.

Air - Rainbow Dash
Flight/Superspeed - Rainbow Dash is a free spirit and impulsive, running on instinct at most times. The wind characterizes this far more than anything else. It flows over and through things with no effort or second thoughts and even if it leaves in a hurry, it’ll almost always come rushing back.

Water - Fluttershy
Healing/Animal empathy - Something we all have within us and can’t live without for long, water. But also kindness. Water is gentle, but can crash with the force of a boulder under the right (or wrong) conditions. It is also dynamic and can fit into just about any niche, if it is of a mind to. Pure water has long been thought to have healing properties and who more flexible, pure and gentle, but with a potential to be harsh, than Flutters?

Dark/Fire - Pinkie Pie
Entropy Acceleration/Transmutation - Weird one, right? Exactly. What is darkness than the most pure manifestation (anti-manifestation?) of ultimate entropy? Pinkie’s powers have been shown to take materials that are high in energy potential (sugary treats) and convert that into work, but not without loss. This is a form of what has been considered ‘alchemy’ in ages past. Pinkie’s erratic, energetic nature is a ‘black hole’ of energy. She is constantly doing something, productive or not, and sucking up even more energy and matter from the world around her to fuel her activity. If more things in the universe were like Pinkie Pie, it would be a dark, cold universe a lot sooner. Still, without darkness, we would not know light. In that way, it brings light to existence. Darkness can also be as much of a comfort as it can cause anxiety, much like Pinkie’s tendency to speak volumes of the things one doesn’t want to hear, but also make them laugh when they didn’t think they could.