There is a certain idea cooped up within my head, and I have no writing skill or any intention to gain any at the moment, but it could prove useful for more seasoned writers wanting to write for this group.

Why not play around with accents, or even languages?

Lets say we have a soul swap between some Irishman and Fluttershy. Right away, a writer can use his lingual habits to reinforce the story. It could be used to make a change noticeable to Fluttershy's friends and arouse suspicion early in a fic (If desired). A scenario like this can also be used to introduce a different perspective from the human due to slight cultural differences between Ireland and North America that are not too big to be cheesy. (Example: Irish-shy sings folk songs to her friends and can take advantage of the poison joke to restore her previous songing voice)

Even within the US there are different accents to pick from for a protagonist that can be used in a way if a writer does not want to deal with cultural differences. (Twilight with a long island accent maybe)

A second idea is Language, but this can be hard to perform well even with all the different communities who speak languages other than English (There's even a Finnish one!) that could help you. This can be used to bring more stress on a story and the signs of a swap can be noticed instantly an example could be a swap between a Russian and RD, with Russ-Dash unable to communicate with her friends properly.
       
A writer can go two ways with this:

 The first way could be to have a the Russian speak broken english, with grammatical mistakes and such, giving a comedic tint to the fic.

The second way could be to have a human with no knowledge of English whatsoever to provide a major handicap to overcome. With this there is a large cultural divide that could be either useful or unattractive to an author. this is almost impossible to avoid going down this route.

I have always thought of a fic along the lines of 'a dash of humanity' where the pony has a voice in his/her head speaking utter gibberish, and even worse when the human has control of the body, where nobody can understand them.

I have no idea how useful these ideas are, but I want to put them out anyway.

5291680 , The problem with trying to write out accents when there's no audio to go with it is that it can make things really awkward for both the writer and the reader. Some accents just can't really be captured in text form alone, and trying to spell it out only prevents readers that have never read that accent in text form before to understand what the writer was going for. Ultimately, that would be a hindrance more than an exciting reading experience.

At my personal best, I could give a brief description of the speaker's accent (or language) while continuing to write their dialogue in non-distinct English.

5292738 You cannot represent the accent phonetically, but what you can do is represent the dialect.

Examples of slang from Irish:

Feck - a minced oath (similar to fuck)

Bowsie - a person of unruly nature

Amn't - am not

There is an entire wikipedia page on Irish speaking habits:
https://en.m.wikipedia.org/wiki/Hiberno-English

There's probably better sites out there but wikipedia works alright.

Just by dialect alone an accent can be implied.