This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
Work on the next standard, predictably called ATA-4, has already started. First and foremost it is a much-needed attempt to merge ATA-3 and ATAPI into one. Faster transfer modes are also on the agenda, of course: it will incorporate Ultra-ATA's DMA/33, at least.
Regarding other features, the future for strong command overlap looks rather bleak: the current proposal is a hideous animal and important players like Microsoft do not plan on supporting it in its present form. More limited forms of overlap stand a somewhat better chance of surviving.