I have a SF short-story idea, but I'm having a problem fleshing it out (terraforming question here)

[NickB79]

I have an idea for a short story in my head that is inspired by some of Stephen Baxter's writings, in that it takes place over vast stretches of time into the distant future. A central aspect of it would involve planetary terraforming within a huge number of star systems spread across our galaxy.

The problem that keeps bugging me is that all the ideas I've seen kicked around about how to terraform dead or near-dead worlds only scratch the surface, literally. Take Mars for example. It has no active, molten core, which means no plate tectonics, no volcanism, and a very weak electromagnetic field. If we were to terraform it with our current ideas (thaw the CO2 polar caps, melt the subsoil ice, bulk up the atmosphere, etc), it may be liveable for tens of thousands of years, but would require constant human intervention to make up for the lack of working hydrological and geological cycles or risk falling back to a nearly dead world. To create a terraformed world that will maintain a rich diversity of life for billions of years requires a molten core.

So, can someone suggest a plausible idea on how to re-liquify a planetary core without destroying the planet itself? Antimatter injections of some sort? Massive artificial gravity systems that flex the planet's core to generate internal heat like we see on Io or Europa? Anything?

Thanks!