I think there are many ways to make a tank more efficient. As previously stated, minimizing heat loss will help a great deal. In some of my native tanks, I keep them in the windows where solar energy heats the water, but this also causes 5-10 degrees of temp fluctuations not suitable for many species. Like others said, use insulation. Instead of painting the back of your tank black, use insulation to create the background. If you are using a canister filter instead of a HOB, it is not that difficult to completely seal(rubber seal, dont silicon it) the tank. If it is completely sealed, you can add insulation to the top and minimize heat loss even further. The problem that creates though, is airation and gas exchange. Live plants help oxygenate the water. If your tank is heavily planted and understocked, you shouldnt need an airator. The plants will also help cut down on waste and pollutants. The plants would also help with the pollutants and might help solve the gas exchange problem of a sealed tank. If you do enough plants and a big understock, you could probably use less, or smaller filters. On some of my tanks I use 3 filters, if i had alot of plants and a smaller stock, my tank would thrive with only 1 filter and no airator. Natives or coolwater species would take away the need for a heater. If you did all of this you could have a tank that only uses 1 filter and no other mechanics.
It was said that it is cheaper to heat the whole room. when I need a heated tank, I keep it in my herp room that is already 80+F from all the heating elements. So I still dont need a heater.
Really, I think it is difficult to make your current tank alot more efficient. But if you plan from the begining to create an efficient tank, it is fully possible to design a highly efficient tank.
Im not completely sure about some of my ideas such as the sealed tank and whatnot, so please offer some constructive criticism on problems I overlooked.