I think we can assume that most users will want to search "incrementally": search, evaluate results, modify query, repeat. The big autocomplete might interrupt the user's query entry process. Not always will a subject heading search be better than a free-text one, so suggesting terms directly might be detrimental to this process. So, perhaps of autocomplete, subject-heading related to the current query could be presented after the search has been performed?
This approach could also make it easier to fit a more advanced interface for selecting subject headings. The hierarchical context of terms, for instance, doesn't easily fit in an autocomplete drop-down.
I think we can assume that most users will want to search "incrementally": search, evaluate results, modify query, repeat. The big autocomplete might interrupt the user's query entry process. Not always will a subject heading search be better than a free-text one, so suggesting terms directly might be detrimental to this process. So, perhaps of autocomplete, subject-heading related to the current query could be presented after the search has been performed?
This approach could also make it easier to fit a more advanced interface for selecting subject headings. The hierarchical context of terms, for instance, doesn't easily fit in an autocomplete drop-down.
Compare to how Ebay shows search suggestions: