Set it way in the future. No Marty... No Doc... and have someone new stumble across "Doc's lost journal" (perhaps a descendant of either Doc, Marty or even Biff), which includes instructions on how to build a time machine. To introduce a bit of nostalgia the producers could get Christopher Lloyd to do a voice-over cameo and read out the instructions to the audience. That way... somebody new could build a new time machine out of a completely new and (hopefully) iconic car.... it could be the start of a whole new trilogy. Where would they go? Easy. The world gets told that climate change and global warming are huge issues and that, if humans keep going the way we are, it'll eventually cause an extinction event (basically something that spells the end of the world). Scared... that person uses the time machine to travel into the future to see whether the prediction was true or not. We'd get a really cool futuristic and possibly dystopian world... and a strong message.