[sevensor]

New materials. The search space in materials science is so impossibly huge that it's an endless source of surprises. The last ceramics hype cycle was about 20 years ago, so we're due for another one. Maybe room-temperature ceramic superconductors?

[bllguo]

I'm biased as I studied MSE but I think this is the best one. All the other answers - electric cars, AI, robotics, etc. - are already dominating headlines; they aren't surprising. A new material could be very surprising. This is a common saying in materials science - notice how the major ages of human development (bronze age, iron age, even arguably the silicon age) are named after materials? Who would have predicted them before they occurred?

Unfortunately, from my experience, the turnaround from finding an innovative new material to actually using it in real products is absurdly long.