I hope this can be resolved; I’m currently looking for solutions to the lack of deadlines and Assignees implemented by individuals at my company. Asana should provide a rule whereby a task cannot be created if certain fields (in my instance, deadline and Asignees) are omitted.
I saw there was a similar post a few days ago re: a rule when a deadline is added to a task and then removed, but that doesn’t account for a deadline not being posted in the first place.
Welcome to the Asana Community Forum @Cassie_Forbes 
Here is an existing feedback request thread I recommend to upvote:
Now in case you require the same for a custom field selection you can set something up via rules similar to what I explained here.
The example I posted was for no task completion before updating a specific field but basically similar can be set up for task creation via comments being added or tasks moved back to a section if conditions are not met.