It would help development to have a bunch of extra compile time checks and warnings for behaviour which is not necessarily wrong (and therefore wouldn't normally spit out warnings), but which might indicate coding errors, like using ints in enum types or the wrong enums. Or using undefined functions/variables (which is valid JS but often just means a misspelling).

This would probably be a configuration option on the engine, only available in debug builds, so it doesn't affect performance (a lot of these checks are far too heavy to do at runtime).