I hope am not too late to answer this question but sometime back I created a news aggregation search engine and during this time a lot of research was done by me in order to try and address "media bias". What i found is that there is absolutely no way to identify bias within the news circles why? Well because we do not have a scale. The best way to put this is to think about images we see on reddit. Think about that image of a car taken from a zoomed in perspective and made to look really large. You might actually assume that it is a real car while it could be a toy car when placed next to a scale i.e. "banana for scale". There is no clear scale for bias in the news organizations and it makes it really hard to determine what is not bias. In addition to this, there is also the issue of context. One story can be taken largely out of context by either editing in or out a piece of information. This makes a story seem worse or better than it ought to be in order to deliver the shock factor and this is also supported by the fact that news organizations do not necessarily have to reveal their source be it data or word of mouth.
There is also the notion that the news may be a monopoly controlled by a couple of companies. In economic terms, I don't think they are a pure monopoly and let me explain why. Anybody can start a news org and all it would take is a laptop and a story. If you have friends, the better but what tends to happen is that most of the public want "trusted" sources. In this case trusted can mean that you have been there longer or are more wiling to offer the public the bias they want to hear. So what happens is that smaller news organizations stop reporting what is necessary and start appealing to what people want to hear. Once this trajectory begins its usually hard to stop because profits start coming in and people love money.
Also when the company initially tried to offer a transparent view on issues, they do not get he support they need. Asking for money from investors will often get the door slammed on their faces or get the question "what are you offering that is new and other news agencies don't do". Most investors don't like the transparent issue as they view it as a naive approach to creating a product. But once the news org starts leaning right/left then money comes in from sympathetic investors who love what they read because it agrees with their bias. The organization does better and eventually drop the transparent agenda that they once touted.
Honestly we can't stop what is mainstream and what is not since they only prove to validate the readers bias. What we can do is offer a solution where all the news stories are compiled and we can create a way for people to sort out what they are looking for while exposing them to different ideas that exist within the same realm. I created a news search engine for that but lack of attention as well as the fact that i did the project by myself made it hard for me to do the marketing, programming as well as promotions. It is a hard task but its very doable.