Please vote for which proposed solution, if any, you think is the most promising potential answer to The Last Question. 

Invent interdimensional travel and bring in energy from other universes.

LET THERE BE LIGHT - Restart the universe with a second big bang.

Giant magnifying glass to concentrate sparse infrared heat radiation into a single point, lighting a fire.

2nd law of thermodynamics depends on conservation of energy. General relativity does not actually globally conserve energy (look it up)

Harness the suffering of Magical Girls as a source of entropy-reversing energy

It can't, but if you could trigger another episode of cosmic inflation, it would cause entropy *density* to plummet, after which entropy could increase to a higher maximum.