The most common oversight made by contractors is failing to consider the depth of existing competition. Profitability is greatly impacted by price pressure right? Few things drive down prices faster than a high number of contractors available to perform the work. This is why focusing on one or two niches often works so well.
Find a niche that requires a significant amount of specialized skill and make sure the field people who have that skill stick around. Niches make marketing easier, they make selling easier, they make operations management easier, all of which leads to higher margins.
The least overlooked issue is the size of the available market. Once a contractor identifies an available market keeping track of its size is fairly easy. Usually there is a small group of A/E firms and / or specialty equipment suppliers who can be networked with to keep a finger on the market's pulse.
One factor I didn't mention, mainly because over the long haul it shouldn't prevent you from adopting a corresponding business model, is the cash flow inherent in the business model. Needless to say, certain models lend themselves to faster pay. Service work is a great example. If you pick the right model(s), run a profitable business and keep cash in the business you should be able to live with a slow pay model. One advantage of a slow pay model is that it limits competition. If you think things are bad in the GC world now, imagine how price competitive it would be if GCs paid every 30 days.
Marketing your model
Branding can become very difficult when you adopt multiple business models. The "all things to all people" marketing message creates doubt in prospects' minds. The incredible proliferation of specialists in the last 20 years has left us with the conclusion that we need a specialist with experience. It's no longer good enough to say you do drywall in commercial buildings. Now you need to say that you've down several remodel projects in operating hospitals.
Take that thought and apply it to your marketing and selling programs. How are you going to successfully convince buyers (including GCs) that you are the preferred choice for their project when you are running around claiming to be great at everything. Nobody is going to believe that pitch even if you are great at everything.
Summarizing, if you are under $2 million pick one business model and stick to it like glue until you are (a) consistently profitable, (b) growing past $2.5 million and (3) willing to implement the costing and marketing systems necessary to support two models.