No offence but I absolutely hate getting put with a team mate that only plays one hero, there's just no way you can fully understand what counters what or how a hero feels or what its strengths and limitations are without playing them. The first thing I did was play every hero once, figured out who I liked and played them some more, then I went out of my way to play my least played heroes and I'm now a better player for it. Being able to play every roll well is absolutely vital, having at least one in every category is a must, preferably 2 or more so you don't counter yourself, for example what do you do as a Pharah "main" if the other team goes double Soldier? If they have any idea how to play you're now a hindrance to the team.
It seems like you want to branch out though which is good, just start by filling the roll the team needs, going through all the heroes slowly, eventually you'll find ones you're best with and enjoy the most and as long as you have a varied stable you'll always be an asset.