Where Dromund Kaas is in the Galaxy is somewhere in the Outer Rim; the planet itself is at the center of a sphere of influence referred to as the Sith Worlds. I'm not sure if the Sith Worlds as a region was wiped from the databanks, but by the time of the Clone Wars one of those planets, Korriban, draws the interest of one of the conglomerates that form the CIS.
Where we are is far from the Republic, but within the relative backyard of Palpatine, Maul, and any Dark Side Adepts that have made their home there. Suppose the Last Elder wants to expand, the long term goal would be to reach all over the Sith Worlds, and see what happens from there. Controlling the Sith Worlds might mean squaring off with every Sith and Dark Jedi who might oppose us. The Jedi might join the fray as well as we'd be a warning sign of the Sith's return, whether or not Palpatine is ready for that kind of attention.
EDIT: I just remembered that we know of Ziost and Korriban, and can travel to those planets when we can do hyperspace.