I'd love to have the Crossway ESV Church History Study Bible Notes. I find bible notes to be a great way to have a (concise, short) commentary available on iPad during church, etc. They don't replace real commentaries, but they do mention some of the important stuff without taking up as much screen real estate.
This appears to be mostly a collection of quotes from historical church figures, which would be a nice addition to those who preach and teach as well.

