I sort of want to add on to this tattoo. I have no idea where to start. It seems like this piece is a completed idea and I am afraid adding on might just look dumb. I was considering getting a second dragonfly going the opposite direction into the sorta empty space. Any ideas??

So I went back added the bird, some yellow to the koi, and evened up the bottom portion and a few minor touch ups. I think I am done now :)

I'm not all that good at imagining things.. but if I were you, I'd go visit your artist with some of the ideas you have and see what he thinks would work best. 

 bamboo?  maple leaves?  those are two things I associate with koi fish.

 what about a lotus? beautiful work by the way.

 Are you considering going for a full back piece?

You could incorporate all of the ideas folks are throwing out here...

I'd consider adding other elements - some earth and air.. and maybe fire... in the japaneses theme.  

Also, I would take some of the elements that you come up with to your artist and see what s/he thinks.

I am a little more than half way through a full-ish sleeve and came in with the two core components but have left the rest of to my artist with stellar results.

 I'm part-kidding here, but you could add a seagull or a Pélican (This wide-necked bird that can fish while flying and keep all his food in his beak, I don't know the english term) attacking the fish, on the up of your back.

(Or a fishline)

Thanks for all of your sugestions. It is now complete.....at least I think it is for now ;) 

 Oh wow, that looks great! It does look complete now. :) Fabulous!

 Nice!

Is that an early bird....

That's about to get...

 the worm?

That is amazing! I love it!

The bird was a beautiful Touch!

 beautiful!  I love the bit of green that the bird added to the piece.