If MAME could support the VT525 (nearly the last terminal DEC made and unlike the previous DEC models it supports ANSI color) people might use it a bit more. It would be very useful for compatibility testing as there aren't many people with a real VT525! Last I looked someone had dumped the ROMs but there wasn't any support code.
I've started working on a VT420 emulator which is about 70% complete: https://github.com/mmastrac/blaze
VT5xx was the budget line with limited functionality, that's why only 525 among them supports ANSI color. The only fancy stuff was multisession (TD/SMP if you have all bits to support it) and "desktop accessories" like clock and calculator.
The really interesting ones are VT340 variants with ReGIS and full SIXEL graphics