There are a couple I have in mind. Like many techies, I am a huge fan of RSS for content distribution and XMPP for federated communication.
The really niche one I like is S-expressions as a data format and configuration in place of json, yaml, toml, etc.
I am a big fan of Plaintext formats, although I wish markdown had a few more features like tables.
(Holocene or) Human Era calendar
That would represent all human history as one.
That sounds interesting, would most likely not be very popular with lots of people and a pain in the butt to implement but interesting.
There’s a cool video from In a Nutshell about it some years ago.
And also, the Dekatrian calendar
Where we would have a less broken, more regular, year calendar that is almost align with the moon cycle.
Oh many years ago in school I created something like that for an arts/creative writing project once, a calendar with 12, 30 day month based on sailor moon. Having it based on a magical girl manga gave me the freedom to declare the rest of the days to “days of evil” Was a fun project because I created a whole religion around it. 😁