The one which I know about and could make a real difference to the world is trying to make nuclear fusion happen. Scientists theorise that it is possible and proved it on a small scale. Now engineers need to make it happen in a way that serves the world and is profitable. In the small scale achievements a lot of exotic and expensive materials were required. If you want to scale it up – these materials would be too expensive or more problematically there may not be enough of the material on our planet to make it happen on a big scale. This is what I mean by turing ideas into something that works in the real world – you have to consider a lot more when developing things for the real world than testing ideas in the lab or in your imagination.
There are a couple of ways people are trying to create fusion – by magnetism (google ITER and JET) or by high power lasers (google HIPER). Engineers are (and have been for about 50 years) to make this happen in a way that will solve the energy problem (running out of fossil fuels, creating nuclear waste etc)