As I'd mentioned way back when, the only major difference between Portfolios here and dA's gallery folders is the ability to manually reorder their display after upload. InkBlot doesn't allow this, and B*zzly never did in the term I was there. Given you added sorting options by artist and title, you've already gone to lengths dA never did; in theory a fourth option could sort pieces based on a user-specified order tied to the folder (so the uploader's portfolio index doesn't conflict with a user's Favourites, for example).
The main consideration, of course, is ensuring such indexes have logical protocols for ordering new additions without discrete positions and/or determining priority for items using the same number.
dA also allows custom thumbnails for the folder icons themselves, but that's not a priority.
(P.S.: Subfolders are a premium feature on dA, so adding them here, Credit-perk or otherwise, is an easy leg up. :^) )