I think if you were to use the movie The Death of Twilight Sparkle as a baseline for who the overall main characters are in Tamers MLP are probably...
>Twilight
>Trixie
>Flawless
>Rarity
>Celestia
>Shiny
>Cadence
>Luna
>Chrysalis
>Sunset
And then with the latest movie
>Mari
>Starlight
>Rainbow Dash
>Maybe Maud
What about Soarin, Big Mac, Braeburn, Apple Slit, and Care Package you say?
Their arcs are over I think they're side character status now.
I don't think its a coincidence that the vast majority of the potential main characters are Unicorns.