To call Apple's Thunderbolt Display long in the tooth would be an understatement. It's so old that the first Retina display Macs were still a year away when it was new, and it's now easy to get a 4K display for less. At last, though, it looks like Apple might be ready to give its standalone screen an upgrade... with an interesting twist, to boot. Sources for 9to5Mac understand that Apple is building a 5K display that includes its own graphics chipset. It'd be an odd move, but it would let many Macs use the display, not just high-end models. If your Mac's onboard graphics weren't fast enough, it'd automatically switch to the monitor's GPU.
The tipsters don't have full details, but it's expected that the screen would use Thunderbolt 3 (which relies on a USB-C connector) to connect both the display and GPU. You probably wouldn't need the multiple video cables of existing standalone 5K screens, then. Also, it'll supposedly be "very similar" to the current-generation 5K iMac's display, complete with movie-grade color accuracy.
There's no mention of a firm release schedule. However, people have noticed that Thunderbolt Display stock is running low at Apple Stores around the world. It wouldn't be a stretch to imagine Apple unveiling a 5K monitor (and maybe a Mac built for it) at WWDC in mid-June, when many pro Mac users will be watching. If and when it shows up, it'll arguably be overdue. Mac Pro users have been crying out for a higher-resolution Apple external display for years -- after this, they wouldn't have to consider switching to an iMac to make that happen.