I shall give you many a theme music piece for a MOC and explain their reasons.
Theme music: Valentin Booves - Evolve
All my Toa Vihagu MOCs are based on real people, some to a greater extent than others. With my first few MOCs they were quite close to the originals. Since Mokatu's real-life inspiration plays the violin, I wanted a violin-centric theme, and this was the best one I could think of. However, I think that for his V2 I want a theme that's a bit faster paced (but still orchestral-based), and at present my choice is Celldweller - The Wings of Icarus.
Theme music: Ninja Tracks - Cloud Pusher
Again, based on the real-life inspiration's musical abilities. Therina's inspiration is a pretty dang good singer, so I wanted a theme that emphasized the choir. The female vocalist in this piece sounds frighteningly similar to the inspiration.
Theme music: Punch-Out! Wii - World Circuit
Two main reasons for this one. Firstly, since Drainu's character is more laid-back and less tense, I wanted a theme that suited that personality in compared to the more dramatic tones of previous musical pieces. Added to that the inspiration's liking for Little Mac in Smash (which carried over to his MOC) and this was what I chose. However, I'm still not really happy with this theme. Since the inspiration is also a Zelda fan, I'm considering giving him a metal cover of "Saria's Song" from Ocarina of Time as his V2 theme.
Theme music: Fire Emblem Awakening soundtrack - Id (Purpose)
Like Drainu, based on the fictional character that inspired the MOC. I was tentative at using a theme that was already another character's theme, so I looked for a similar-sounding theme by trailer music group E. S. Posthumus (which specializes in ancient and non-European styled music) but ultimately had to go with Id.
Theme music: Celldweller - The Angel of Io
This was one of the more arbitrary themes. I knew that to emphasize Ayen's character, and the duality between her light and dark forms, I needed a theme that was soft but tense. Angel of Io was the one that best fit the bill for me.
Theme music: Audiomachine - Nemesis
For reasons too complicated to explain here I associate this piece with fire, Zerkah's element. That, combined with the emphasis on guitars (again, the instrument of choice of Zerkah's inspiration) settled him.
Theme music: Peter Roe - Way of the Sword
Originally, I was going to go again with the theme of the character that provided the inspiration of the MOC (in this case, "Gerudo Valley") but was again tentative. I had heard "Way of the Sword" a bit earlier and thought it fit, given Saidon's less violent nature and the fact that it emphasized his weapon of choice.
Theme music: Skylar Cahn - Titan
Again, emphasizing Shaju's instrument of choice (the electric guitar), and with a title that describes Shaju pretty well.
Theme music: Brand X - Halfway Home
Firstly, this was a piece I fell in love with when I first heard it and quite wanted to use on a MOC. Secondly, the theme's title and the content of the piece itself reflected Jetera's arrival in a place "halfway to home" and his vague memories of the past.
Should probably stop there, but I'll give a little teaser of Jetera's V2 theme: