I often wonder if we can seed Venus with a large collection of extremophiles from various earth environments? Like various hyperbaric sulfobacteria etc at hydrothermal vents at the bottom of the oceans and volcanoes. Some of them might survive and start the 'terraforming', even if the process takes several millions of years. Not that they can make it human-inhabitable anytime soon, but we can observe what microbes do survive, and figure out a way remove the sulfuric acid etc. And if any of them start thriving, they can be surprisingly fast in colonizing the planet.