Unfortunately, the way you want it to happen and the way it happens are not lined up. You cannot change the template with workflow until the change registers with the subscriber to activate your workflow, which happens when you save and exit.

I also have wanted dynamic fields like what you describe but I understand why they might not get around to it for a while. It's too hard to handle this type of task and keep it lightweight. That type of processing requires a lot of information to be processed and sent to the machine with the client open and can easily become unstable if you have too many conditions. 

If you have LF Forms though, you can do something interesting like this though. You can use forms and it's page rules to hide fields and then use workflow to input that information into the documents metadata.