I agree the stock is nice as it is. A good mix of compatible fish that are likely to work for the long haul.
If I were to add anything, it would be a few more Columbia tetras and that's it.
I do thing a tangle of thin branches coming in from the top corner was add a lot to the aesthetic and as Neil pointed out it would give the tetras a sense of security and show them off better.
IMO a good sized school of tetras in an amazon themed tank can really steal the spotlight and add enormously to a display