@FaheemMitha the reason that LaTeX3 has taken its time (and still does) doesn't have to do with a choice of coding approach (whether that being TeX internal based, or Lua, or Phython or what have you) but with the problem it intends to tackle - solving those is the issue and they aren't solved by dropping in a different interpreter. Have a look at
latex-project.org/papers especially "E-TeX: Guidelines to future TeX extensions", or "The stony road to complex layout" and a few others