I'm waiting for Phase 2[1]: especially "Algebraic data types & pattern matching". Current tagged union[2] use is pretty awkward/verbose and lacks exhaustiveness checking and typed error handling.
Extensible trait support seems like another big one if the ecosystem is going to significantly grow. Not having clearly defined traits in the language made larger projects composing many libraries in Julia difficult.
[1] https://mojolang.org/docs/roadmap/#phase-2
[2] https://mojolang.org/docs/std/utils/variant/Variant/