Plane 2, the SIP (Supplementary Ideographic Plane), is almost full.
Right off the bat, in Unicode Version 3.1 (March of 2001), Extension B filled it nearly two-thirds of the way with its 42,711 characters, along with 542 CJK Compatibility Ideographs. Extension C with 4,149 characters was added in Version 5.2 (October of 2009), Extension D with a mere 222 characters was added in Version 6.0 (October of 2010), and Extension E with 5,762 characters was added in Version 8.0 (June of 2015). On tap for Unicode Version 10, scheduled for a June of 2017 release, is Extension F that currently includes 7,473 characters (U+2CEB0 through U+2EBE0).
The following is a colored representation of Plane 2 in its current state, with unassigned 16-character blocks left blank (white), and including Extension F in blue:
If Extension F remains stable—at least in terms of staying within its current block, specifically U+2CEB0 through U+2EBEF—that would leave only 4,622 unassigned code points in Plane 2. These code points are broken down as 32 in the proverbial no man’s land between Extensions B and C (U+2A6E0 through U+2A6FF), 3,088 between Extension F and the CJK Compatibility Ideographs Supplement (U+2EBF0 through U+2F7FF), and 1,502 immediately after the CJK Compatibility Ideographs Supplement (U+2FA20 through U+2FFFD). While there is a non-zero chance that what will likely become Extension G may squeeze into Plane 2, there are no guarantees.
Luckily, there has been a rough plan in place to open Plane 3, the TIP (Tertiary Ideographic Plane), which includes the usual set of 65,534 code points, from U+30000 through U+3FFFD (remember that the last two characters in each plane are noncharacters, which means U+3FFFE and U+3FFFF in Plane 3). As the TIP roadmap page indicates, it is intended for encoding scripts from the Old Hanzi Period, such as the Small Seal and Oracle Bone scripts, but that may change as soon as Plane 2 (SIP) can no longer accommodate additional CJK Unified Ideograph extensions.
Anyway, this is definitely something to keep an eye on.
Updated on 2016-05-26 to reflect the latest Extension F figures.