The best replacement for xaml is xml.

The best alternatives to xaml are rated as following:
★★★★★  xml
★★★  wpf
★★★  qml
★★  svg
★★  windows forms
★★  xul
★★  xoml
★  baml
★  mxml
★  expression blend
  aspx
  winjs

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

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



Google Trends

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