Prediction? I think we will see "performance" HDDs die out. They cant compete with SDD. And at slower speeds, HDDs can store at higher density at lower power.
Next step? 4500rpm 5TB HDD 3.5" format. With slower read/write speeds. Whereas SSD will get faster, and the standard size will become 320MB rather than 80/160 as it is now.
10TB? While in theory you could manuf. a double height drive like the old SCSI drives... due to manuf. and reliability costs (they were horribly unreliable and prone to damage from small knocks)... more likely peeps will just RAID array them.