One long running research area that I'm reminded of is artificial photosynthesis. Let's make a combination PV cell and electrolytic cell, because optimizing both together is going to be totally easier than doing it separately. /s

From the perspective of a career minded professor sitting on a funding committee it's a good way to guarantee a growing set of citations. As long as people still research X the first papers in X will garner citations.

Oh like masers and lasers you mean ;-)