Let's assume that they have full control over congress, so politics isn't an issue. I think looking at what a good global climate policy would be useful, because it allows us to see where we stand. It could also serve as a platform for future candidates.
It seems to me that the new president should take a wide-ranging series of measures to curb emissions in all the major domains: electricity, transportation, agriculture, manufacturing, etc. . You might argue that measures taken in isolation from other countries are not sufficient. While that's true, someone has to start. The US taking the lead on climate change would have a profound impact on all other countries. The US could use its very strong diplomatic weight to pressure other countries to adopt similar measures.
So what should these measures be? The major one would seem to be a carbon tax, applied to all major sources of emissions: energy production (coal plants, ...), agriculture (cattle and meat imports), jet fuel (current taxes are very low), etc. Another one could be a tax on imports depending on how much the exporting country does against global warming. Maybe a new kind of free trade alliance among "climate-virtuous" countries could be created.
Any thoughts? Have any serious global policy proposals been made and studied in the past?