Minimi: I think the melody is very nice already, you don't have to add another one. And about the instruments, as you've changed them all to strings, you could add another instrument (or instruments) to double the melody. The same goes for the harmony. It might sound nice with a solo wind instrument, or a french horn an octave lower for the melody. And arpeggiated chords in the harp might be nice as well. You obviously don't have to, but sticking to standard orchestral instruments would probably sound the best.
And about making it longer, you can repeat the melody with different orhestration, or just start a new idea (but similar to the first). And you could play with the dynamics too. It might be cool if this slow mysterious theme crescendoed into a more grandiose theme (which you could call the main theme), or whatever might suit the overall atmosphere of the game. I dunno, just ideas. Good luck!
And about making it longer, you can repeat the melody with different orhestration, or just start a new idea (but similar to the first). And you could play with the dynamics too. It might be cool if this slow mysterious theme crescendoed into a more grandiose theme (which you could call the main theme), or whatever might suit the overall atmosphere of the game. I dunno, just ideas. Good luck!