ComputerWorld is reporting on a roadmap from the industry consortium ASTC where they predict magnetic hard drives will reach 100 TB by 2025. This is based on the figure below, where they project data areal density will increase a ten-fold from today's 1 Tbit per square inch.
The technology to make this work, is Heat Assisted Magnetic Recording (HAMR), which integrates a laser into the hard disk read/write head. Bit patterned magnetic recording (BPMR) will take this further by isolating the bits into smaller "islands". Finally, Heated-Dot magnetic recording (HDMR) combines the two technologies to reach 10 Tbit / square inch.
However, it is unclear if these technologies will be compatible with the helium filled drives HGST released earlier this year. From the chart, it seems the new technologies are partly based on Shingled Magnetic Recording (SMR). If this is not compatible with seven platter helium drives, we are stuck at only five platters. The biggest of this kind right now is Seagate's 8 TB disk, but that is already at about 2 Tbit / inch. Therefore, the increase will "only" be five-fold, starting at 8 TB, for at total of 40 TB in a 3.5" drive. A four times increase of today's biggest drive in ten years would not be very impressive.