What kind of RPG is this? Realistic graphics or sprites? For a main theme, On the Horizon feels like it kind of wanders around a bit musically. After hearing this once, I'm not sure I could hum the theme a few days later. Contrast your piece with main themes of big name video games or films. The opening track hammers the main theme into your head over and over. It's very solid and has a clear, concise message.
Just for an example: Harry Potter main theme
Listen to how many times it returns to the main theme, in different sections of the orchestra. Each time it's restated, the listener's ear has another opportunity to latch on to it. Now Williams does leave the main theme for some related, expositional material but he returns to it, somewhat, later on.
So, for your main theme, I would revisit your melody and then explore ways to restate it multiple times so it sticks more. In fact... to be honest, after having typed this message, my ear has already lost your main theme. There are many other things we can discuss about production and the actual samples used but focusing first on the notes and the music structure is best.
One of the best bits of advice I've read about scoring was to create a great main melody then restate it, restate it, restate it.
I hope that helps!