S'pose you realize you're in the market for an embedded DSL that, let's say, controls a natural gas pipeline system. One approach is just to start writing things you want the language to be able to handle--for example,
set valve(Pittsburgh,18,1349) open
puts "Flow in critical link is $flow(Valdosta,7,88)."
if [string equal open ...But wait! This is valid Tcl already! Let's figure out what the output from such a sequence should be, and we immediately have a case that can be coded directly in tcltest. Now all that remains is to implement (just) enough to pass the test we've composed. This is TDD with a vengeance!