The best replacement for trackball is mouse.

The best alternatives to trackball are rated as following:
★★★★★  mouse
★★★  trackpad
★★★  touchpad
★★  laser mouse
★★  mouse gaming
★★  vertical mouse
★  mouse ergonomics
★  mouse carpal tunnel
★  optical mouse
  trackpad blackberry
  normal mouse
  track ball
  trackpad better

Your alternative for trackball is not listed? Suggest!
Speaking about trackball, one probably shouldn't forget about scroll.

Other search terms related to trackball and their ratings are:

★★★★★ scroll
★★★ keyboard
★★★ scroll wheel
★★★ keypad
horizontal scroll
You have in mind a companion for trackball and it is not yet here? Propose!

Google Trends

Scale is based on the average worldwide traffic of trackball in all years. Click on graph for more.