To be honest very few scientists do anything as big as an Einstein or Newton. Most of us work in teams and some of those teams do something really big, for example the human genome or the Higgs boson. While there can be thousands of people they are all needed.
I hope I can be part of the team that makes production of fuel from carbon dioxide using light a reality. Thst would be really big.
As Michael said scientists are very unlikely to do anything big on their own but I think everybody hopes to be part of a team that will achieve a major scientific break through. Whether that happens or not I think is a matter of luck and timing – luck to be working with an interesting group of people and timing so that your break through can come after lots of smaller steps from the work of people before you.
I do hope to be part of one of these breakthroughprojects some day but I don’t know what the “something big” might be!