What engines LaTeX3 is (and will be) designed to work with ? Like 
LaTeX2e it will work with TeX, PDFTeX, XeTeX, LuaTeX, (Aleph...???), or 
with a more limited list such as LuaTeX and XeTeX ? I'd like to know if 
a decision has been taken and whether it has, I'd like to know why such 
a decision.

Thank you.