Ok, so let's look at your music here.
You have the G F# Fnatural E bassline section as an introduction.
It's roughly a G chord, D65 chord, G42, C6 progression that happens 4 times.
That's your introduction.
Lets label this A
Then you have. C, D, G6, Eminor, C, D, C64, G. A nice catchy melody as well.
Lets label this B
Then we have A again with a melody. Let's label this A'
So you have A B A'
What can you do?
There are a lot of options but let's kind of keep it simple here. What are you going for and what sort of a sound do you want? Any ideas? Do you want the tune to loop like in a videogame? Or do you want a definitive end?
My first suggestion is to do something like bring back B. You can repeat it verbatim or better yet, why not make it slightly different?
So
A (Intruduction)B A' (A with melody) B (write this)
If I was to write a second B section or a B' I'd use the chords
C, D, G6, Eminor, C, D, C64, Em for a deceptive ending right on the downbeat. Then I'd continue the E as a bassline for a measure or two.
This is a transition that can go a number of ways.
options
1) Transpose the opening part to the key of A instead of G and put a solo over top. Then you could go into B again and transition at the end back to G and loop the piece.
2) Go into a bridge (New contrasting section in E) Then from there you'd go back into B twice (Do some variations for fun) and then you could end the piece.
3) Use the bassline on E to walk down maybe with a riff to C and go back into B again. This is a great way to do an ending.
4) You could walk the bassline down to C and do a bridge in D major or D minor.
So I'm suggesting something that is similar to a song form although I have plenty of other ideas as well.
So far.
A B A' This is what you have.
A typical song would continue with B or B' followed by a bridge C, and then back to B and B would happen twice.
so
A B A' B C B B
Or A B A' B A'' (solo in new key) C bridge B B
Or
A B A' B C Bsolo B'
Just some ideas to get you thinking. Experiment and see what works. See what you like. I hope this is helpful.