Terraforming mars is plausible, and not outside our technological might right now, just out of our economical grasp. If you got mars up to a similar atmospheric density as Earth, it would take solar erosion ten thousand years to erode it. So any process faster then solar erosion would make it doable. The solar system is very abundant with the gasses needed terraform Mars, and it has enough water and mineral wealth to make itself self sustaining. It just a long process, multiple centuries long process given the few postulated methods. It still much faster then Venus, as its much easier to add gasses then to take away gasses. You'll need a gravity siphon, which would require something along the density of a neutron star to work. And oddly, you'll need a something like that to fix the gravity issue of mars, but letting an object set on the surface and sink to the core.
The major issue with mars, is its low gravity, and lack of magnetosphere. The magnetosphere issue can be dealt with shielding areas with pulse magnetic fields.
Extra solar activity will probably never happen, at least never happen beyond the earth luna system. Its just really expensive, and dangerous. It much cheaper to build space stations for any habitation needs or industry needs, or placing those on luna, then it is to place it on mars. Its much cheaper to bring an astroid into Earth or lunar orbit and mine it with access from a space elevator then go to the astroid and ship the goods back.
Even if it does, it'll probably never expand beyond inner astroid belt, or possible the Jupiter moon system. The distance is to vast to make it worthwhile for a human endeavor.