It would be great to create a grouping of fields that would dynamically duplicate on the front end. For example: if someone is registering several people for an event and the form is gathering the same information for all those people (name, age, address, phone, email, etc), being able to create a single group of those fields on the back end inside a subform or repeating section would save SOOOO much time. And on the front end the user would click an "add" button for however many people they're registering. Not only would that save us from duplicating dozens of fields and creating dozens of logic rules for showing/hiding them, we would have to guess at how many people we'd have to create fields for.
When I search for "sub forms" for Machform, all I see is a post from 6 years ago saying they don't plan to implement it. If this feature has already been requested (I can't imagine why it wouldn't have), I can't seem to find it. I find myself needing this feature so often that I end up having to use alternate form systems that support this feature.