You will need another 11-speed cassette that works with a Shimano (or SRAM) freehub. You can go with different sized cassettes depending on the type of riding you do. The sizes are shown as the smallest and largest cog on the cassette. For example, 11-28 or 11-25 or 11-42...and lots in between. As the larger number goes up, you will find bigger jumps between each cog (and much easier pedaling well suited for very steep climbing). Many conventional road bikes with 11-speed gearing come with an 11-28 cassette from the manufacturer, which is a pretty good happy medium for both flat and hilly terrain. It's also important to note that 11-32 is about the limit for 11-speed Shimano road groupsets (the range of the rear derailleur is the rate limiting factor). Check out some of the online retailers to see the different sizes available and pricing.