The best replacement for uart is spi.

The best alternatives to uart are rated as following:
★★★★★  spi
★★★★  usart
★★★★  rs232
★★★★  i2c
★★  ttl
★  sci
★  ftdi
★  jtag
★  gpio
★  dline
★  serial port
  lin
  usi

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

Other search terms related to uart and their ratings are:
★★★★★ fifo
★★★★ dma
★★★ pic
★★ microcontroller
★ irda
You have in mind a companion for uart and it is not yet here? Propose!



Google Trends

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