The closest this franchise has gotten to a theme song is the title song (or cinematic intro) for D1. Destiny (in the future) needs something new and original that makes its presence known. Every great franchise has one and the musical score for D2 is one of the best things about it. Now all we need is a tune or song that is reoccurring and can be the staple of the franchise!
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It has a theme... it's in the soundtrack.