Can we modify/enhance the existing out-of-box visualization in QlikSense?

The out-of-the-box visualizations aren't open sourced. You will have to build your own Qlik Sense extension or use one that is open sourced on branch.qlik.com/github. Another option if you find the Pivot Table under-powered you could have a look at what Vizlib is offering maybe that is what you are looking for although you will have to wait for Q2 2018 to get a specific Pivot Table.