There are no concrete display rumors that have hinted at Apple's plans for the Thunderbolt Display, but there are a couple of different upgrade paths that Apple could go with should it choose to introduce a new display product.
Apple's displays generally share the same screen as its line of iMacs, so it's possible Apple is working towards introducing a 27-inch Retina Thunderbolt display that offers four times as many pixels as the current Apple Thunderbolt Display, coming in at the same 5120 x 2880 pixels seen on the 27-inch Retina 5K iMac. A display at this size would be able to offer the same amount of screen real estate as the current display but with a much sharper appearance, using the same technology as in the Retina MacBook Pro.
Unfortunately, there are technology constraints that would prevent a 5K Retina Thunderbolt display from being an entirely feasible option until 2017 when Intel releases its Cannonlake processors.
An alternative goal would be "4K" resolution of either 4096 or 3840 x 2160 pixels. Apple has touted its 2013 Mac Pro as being able to drive three such displays simultaneously, leading to speculation that the company is planning to launch its own 4K display at some point. A 4K 27-inch display would, however, require Apple to source displays that are separate from its iMac displays, a move the company may not want to make in favor of holding out for 5K support.
Over the years, 4K and 5K displays have become increasingly more affordable as the technology has become more widely adopted by consumers, so there is less of a price barrier preventing Apple from introducing a 4K or 5K Thunderbolt display. Though prices have steadily fallen since 2013 when the first 4K Macs were growing in popularity, Apple has still opted to hold off on offering a refreshed display.
A July 2014 rumor suggested that Apple had a 4K desktop in the works, which was said to be a new iMac, a new 4K display, or both. The 4K desktop was said to be slated for a late fall release to highlight OS X Yosemite, but did not materialize.
Another rumor suggested Apple could release a new 5K ultra-high definition 27-inch monitor later in 2014, with a resolution of 5120 x 2880, but that also did not happen.
Ahead of the 2016 Worldwide Developers Conference (WWDC) in June 2016, Thunderbolt Displays are out of stock at some Apple retail locations, sparking speculation that a refresh is in the works. It is, however, unclear as of yet why Thunderbolt Display stock is fluctuating in some places, and there have not yet been any rumors to confirm that an update is planned for WWDC.