I think the actual mass of Mars may have something to do with the lack of a magnetosphere. The core of the Earth is very likely radioactive which would keep it nice and warm liquifying the inner layers of the planet. The liquidation plays into the creation of the magnetosphere. I also think that the Earth is massive enough to put additional heat and pressure on the inner layers of our planet.

Mars has some radioactive processes keeping the core warm but not liquifying the core. It's hot enough for volcanoes but not hot enough for plate tectonics.

My idea is that if we could increase the mass of Mars we might be able to put enough pressure on the core to liquify the inside of Mars. Maybe using an orbital ring to drop chunks of asteroids or comets onto the surface would work. Do this correctly and we could spend up the spin of the planet. This way it's day would more closely match Earth's.