I am a firm believer in offering just one service…if you can. A lot of contractors want to do everything because they are afraid of missing out. My contrarian point of view (which has been shown to succeed) is to specialize. If you can be the best basement finishing company in your area, do it.
If you like doing bathrooms because you like the cool feel of tile, then go for it.
The benefit of specialization is you become known as the defacto expert in that market for that job type. You get pricing breaks on supplies since you will buy less of everything, but more of some things.
Finally, being the expert in your market will protect you from the deeper-pocketed competition when it comes…and it always does.