Something I've found useful when perpetrating custom languages is to, as early as possible, bring up a REPL that handles multiline expressions sensibly.

This (a) requires me to build stream/incremental parsing (b) gives me a tangible feature out of doing so that makes my life better (c) means from then on even if the production uses of the language are all batch shaped to begin with I'm regularly dogfooding the non-batch code paths.

(because yeah, it -so- does hurt trying to retrofit that later on, so 'finding a way to trick myself into -wanting- to build it in early' was rather a win for me ;)