1> In 2025 car companies will build cars out of carbon. They have made some kind of deferred to decision to start building cars in 2025. 2> In 2025 car companies will be building cars out of carbon. They'll be in the middle of building cars in 2025. Most like already started.

In 2025 car companies will build cars out of carbon. /They might do it. SIMPLE FUTURE TENSE. In 2025 car companies will be building cars out of carbon. /They have decided to do it for sure. FUTURE PERFECT TENSE.

都可以。将来进行时就是对事实的简单陈述，而一般将来时带有一定的感情色彩。 汤姆明天将正在割草. Tom will be cutting grass tomorrow. 　　汤姆明天愿意割草. Tom will cut the grass tomorrow.

你这个问题应该是犯了一个chinglish 的毛病。关于In 2025倒是一段时间（a period of time） 还是 时间点（at the time）的问题，表述很不清楚。就In 2025单纯来看，应该是时间点，在汉语表述中In 2025 car companies will build cars out of carbon. 可能更加符合你的思维。