I suppose any attempt at terraforming has to create oceans if we want an Earth-like habitat (theoretically this could be achieved by re-directing enough comets

I suppose any attempt at terraforming has to create oceans if we want an Earth-like habitat (theoretically this could be achieved by re-directing enough comets at Venus or Mars provided we can utilise that kind of energy in the future). Could perhaps a form of sulphur-fixing bacteria be developed to colonise these oceans and remove it from the atmosphere as sediment? As for the magnetic field, if the lack of convection in the core is a problem (perhaps due to the core completely solidifying) couldn't we just dump enough heat energy into it to restart the dynamo action? In the future it's not completely unrealistic to think antimatter production and storage will improve, that's certainly a way of releasing absurd amounts of energy.