The pigeon is ubiquitous in the world's cities due to a similarity to the native habitat of these birds -- seaside cliffs in the middle east. We humans went and built these cliffs everywhere, and even leave around half-eaten sandwiches for sustenance. If there weren't cars and cats and predatory hawks, pigeons would think cities were perfect.
One thing that annoys me about the city is litter. I often see little bits of garbage thrown into some corner where it will just kind of sit around for all eternity. Occasionally I've gathered it up and thrown it away myself, but within a week's time, it's back. This gave me an idea.
Using the de-extinction technology I mentioned earlier, I make an artificial variant of the pigeon. This species will have digestive enzymes that can consume paper, plastic, and styrofoam. I'd like to include glass, but glass is made of pretty much pure silicon dioxide, and there are limits to what protein can accomplish. I engineer 20 of these, and release them in a major city, ideally one with an extreme litter problem.
The garbage eating pigeons will clear the streets quite handily. While existing pigeons will go to extreme lengths to grab old bits of bread and discarded lunch things, such as jumping into dumpsters, charging across five lanes of traffic, and I even saw a pigeon try to divebomb a sandwich out of someone's hands once. (This failed.)
So when pigeons can eat stuff that's just lying around, I imagine it'll be snapped up in a matter of weeks. At which time they will move on to dumpsters and landfills, lowering disposal costs.
Of course, there's a catch. No organism is 100% efficient, and birds poop. Birds in fact have an annoying instinct to poop into puddles to disguise their trail from predators, and when airborne often confuse shiny cars with puddles. This is going to mean a greatly increased bird population in the city, and with it, greatly increased car washing will be required. I may be able to breed a new instinct into them to poop into grass instead, which would fertilize the grass.
And if they get too numerous, there's another creature from their native habitat that also does well in cities, the Peregine falcon. This is the fastest moving bird on earth, and exclusively eats other birds. It enjoys pigeons for dinner the way that I enjoy a medium-well steak. Each released falcon will eat a minimum of five pigeons a day.