I have a particular requirement when shopping for e-readers.
I love using Calibre on my PC, and my favourite functionality is Calibre's ability to add custom
online dictionaries. For instance, when reading Tolkien novels, I would add a Tolkien wiki as a "dictionary", so I can quickly get more context about certain characters or events or places when reading. When reading Malazan novels, I would add the Malazan wiki as dictionary. When reading ancient/classical Chinese books, I would add zdic.net and the "Multi-function Chinese Character Database" by the Chinese University of Hong Kong as dictionaries.
Does any e-reader hardware have this functionality?
Screenshots for how the custom online dictionary function works in Calibre:
https://imgur.com/a/oM3U8pg