The best replacement for freertos is ucos.

The best alternatives to freertos are rated as following:
★★★★★  ucos
★★★★  openrtos
★★★★  chibios
★★★  micrium
★★★  rtx
★★★  qnx
★★★  nuttx
★★★  vxworks
★★★  ecos
★★  threadx
★★  keil rtx
★  rtems
★  contiki
★  mqx
  safertos
  tnkernel
  coocox

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

Other search terms related to freertos and their ratings are:
★★★★★ lwip
You have in mind a companion for freertos and it is not yet here? Propose!



Google Trends

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