This is a topic too big to explore with forum posts, but it is primarily a matter of composition, and secondly a matter of sample usage.
The "epic" trailer style is derided by more traditionalist orchestrators and composers precisely because originality within it is very difficult given the defining characteristics of the genre are quite rigid, and secondly: It takes the "pop song for orchestra" approach, which is not idiomatic to the ensemble and therefore winds up not sounding very orchestral at all.
This manifests here (and in other tracks in the style) in the string ostinato, which is an attempt to apply the "chord progression" pop song, piano broken-chords style writing to the accompaniment/harmony in a way that dodges sample limitations.
What's happening is that your accompaniment is constantly moving; jumping around in pitch against the brass line, but it is not moving homorhythmically, nor is it moving parallel, oblique or contrary to the main line. It also sounds like it is in much the same range as the brass, and thus: The brass line is muddied.
The other thing is that it sounds like you have everything going full blast at all times. If everything is loud, then nothing is loud. So the dynamics need to be tweaked overall, but the melodic lines need more crescendo and decrescendo; with a sustain line, you want to ride the modwheel a fair bit.
The next problem is that the theme is totally forgettable because it is exclusively legato/sustain, so it lacks a sense of pulse and the rhythmic structure of a melody is one of the most vital aspects, especially for anything jaunty or what I would call "epic". For example, if you take your keyboard and you play the C major scale descending, one note at a time, it just sounds like a boring scale. Do the same, but add the right rhythm to it and you get "Joy To The World", one of the most famous melodies out there.
In short: You need to use a combination of short and long articulations, and certain notes need a clear emphasis in attack to determine the pulse and pattern clearly; something that just can't be done with exclusively-long notes.
But this is an extremely common problem with modern virtual orchestration: Using different articulations to create something that sounds musical since most libraries are recorded in a very dumb manner that doesn't create much homogeneity between samples. What you will be forced to do is layer articulations like a staccato over the legato patch, but the success of this varies greatly from library to library.
It took me almost 15 years to find a combination of libraries that really work well together for writing the 20th Century Styles I specialize in.
If it were my piece, I'd probably do the following:
• Remove the string ostinato as accompaniment and instead choose harmony according to each individual note of the theme and give this a separate rhythm that contrasts but supports the main theme and plays at a softer dynamic
• Make the bassline work contrapuntally to the theme, utilizing a lot of contrary motion. A pedal tone could be tasteful in spots.
• On first introduction of the theme, have the entire ensemble quieter and double the brass, either in the same section or different, but I'd utilize sixths, thirds or octaves. If it were solely brass, parallel fifths in spots could be good too.
• Add more attack to the melody by layering in some short notes, maybe even doubling with pitched percussion.
• Add call and response and other flourishes and orchestral flair during the last couple beats of the sustained notes in the theme to create more motion and interest
• Have different parts of the theme stated by different sections and doublings, within the same phrase.
I hope some of these ideas help.