The most useful approach for me would be the use of an iPad Scrivener version for proof reading and annotating manuscripts. I would think of it more as a companion, a complement to Scrivener, and not a slimmed down version of it.
Scrivener’s strength is to process long documents, and the iPad is really not well suited for writing text, at least long texts. I happen to use iA Writer for short notes and I believe it is the best thing I could find for writing on the iPad, but it would be a real pain writing more than a few paragraphs, even with an external keyboard. Instead that iPad is very good as a reading device. Its strength is the portability.
So I believe the most important feature for an iPad edition of Scrivener would be one suited for reading and annotating the manuscript. It would be great if I could carry my manuscript on the iPad, annotate it, preferably with a stylus, and then import or sync it in a folder in scrivener for the actual corrections, the real “heavy writing” process.