There's always been a lack of earth pony OC's in the fandom.
Personally if I'm writing a story based on my OC, I'd have him be an earth pony simply because it provides more challenges. For example, if a pony encounters a dangerous monster in the Everfree Forest, a pegasus can simply fly away and a unicorn could use his/her magic to beat or escape the monster (provided the unicorn can do more than just levitation), however, it would be much more of a challenge for an earth pony since they don't have any special abilities, making the situation that much more exciting.