I imagine that no one is responding because many fish are available and fish beauty is a very personal thing.
I will try to give you some ideas even though I am not familiar with the danios and barbs.
It appears that you are "stocked up on" "mid to top depth in the tank" fish with two exceptions.
Via the addition of 2 more serpae tetras and 4 more head and tail light tetras, IMHO, your enjoyment of your tetras will increase due to their increased shoaling. If you have rock and/or wood caves and not a glass bottom then several plecos would be a good addition
You can peruse pleco's at http://www.planetcatfish.com/catelog...de=l&thumbs=16
I would recommend that you procure several of the same species and of a species which are omnivores but principally carnivores.
I you find a few which you like then
1) check on their availability in your area and
2) post which ones you considering procuring as members of the Forum may have experience with and be able to give definitive guidance with respect to these species. If you have some plants and not a glass bottom then cories would be a good addition.
I can recommend five sterbai. http://www.planetcatfish.com/catelog...species_id=316 I you do not have a glass bottom then loaches would be a good addition
I can recommend 3 Yoyo loaches based on experience. http://www.loaches.com/species-index...earchterm=yoyo
I can suggest 5 Sewellia lineolata based on the literature.
Please note that I have 6 of these on order. http://www.loaches.com/species-index...ia%20lineolata