danios are schooling fish...so are white cloud mountain minnows...and hatchets...and most of the smaller surface level fish.
I would fill out your schools of tetras, then add another school of surface level fish, or, you could always go with a couple gouramis...gouramis spend most of their time at the top of the tank, and make very nice centerpiece fish, they also tend to intimidate tetras and other schoolers, meaning the fish will stay in tighter schools, instead of them spreading everywhere. some of the show tanks I have seen around your size have a couple of moonlight gouramis and they accent the tank beautifully.
Be carefull if you get white cloud minnows though, because they like cooler temperatures, while most fish are comfertable at 75 degrees, they are comfertable more in the 65 degree range...they can survive temperatures up to 78 degrees, just dont expect them to live long.
as for bottom feeders, I would stay away from the larger plecos, they get way too big, they are also poop machines, it will outgrow or pollute your tank way too fast.
My suggestion if you want a pleco is to get the bristlenose variety, these only grow 4" or so, making them a little more managable in smaller tanks, and they are very common, most fish stores have them or can order them with ease.
also, to help keep the bottom of the tank clean, get a school of 7 or more corys, they will snarf up the left over food that sinks to the bottom and add some entertainment to the bottom of the tank. stick to the albino,bronze,or peppered cory varieties, much less finicky about water parameters and darn near bullet proof!