To some extent that's already implemented in the form of the various tabs - if you want very recent results, go to the News tab or click the Tools button and select a time range. If you want academic results with citations, go to Google Scholar. If you want information from books, go to the Books tab. If you don't know/care, the main tab will mix results from all tabs.
You can add three more buttons on top of the tabs that are already there, but would their value for more advanced users be worth the extra friction/confusion for average users? And how would it scale if someone wants a fourth, fifth, or tenth "lens"?
You can add three more buttons on top of the tabs that are already there, but would their value for more advanced users be worth the extra friction/confusion for average users? And how would it scale if someone wants a fourth, fifth, or tenth "lens"?