If you bought a dedicated mountain bike, would it also allow you to ride more areas/trails than you're currently riding? If so, and that interests you, I'd say get a mountain bike. They're fun and allow you to get away from the paved world. Just don't get an inexpensive full-suspension rig (ugggh). You can get an aluminum hard tail with a suspension fork (lots of used options, btw), or forego suspension altogether with something like a Surly Karate Monkey 27.5+ or Kona Unit X (or Surly Krampus if you're tall/powerful). Then your second bike can be a fast asphalt-oriented machine with drop bars. Perfect complement.

Or you could go the "two sets of wheels" route that's becoming a more popular option (choice of 700 road tires or 27.5 mountain tires) like the Diamondback Haanjo EXP. But a wheel swap doesn't turn a road bike into a mountain bike. The "one bike" marketing is just that. IMO...