One of the best Speccy emulators, ZEsarUX, features support for the Spectrum Next. The emulator by Cesar Hernandez is already updated with the current WIP hardware sprites implementation, 7MHz mode and SD card support, becoming a solid platform to start developing for the Next right away.
Cesar has been keeping the emulator updates often, implementing any new Next features very quickly. Since the Next is constantly evolving, make sure to update the emulator all the time.
Below is a great tutorial on how to download, install and set up ZEsarUX for use with the hardware sprites. Follow its steps and get going!
Video credits: ChuckBleedinNorris
Download ZEsarUX (external site)