Sorry I'm ootl. What's The Big Wait?
It's a nickname for the next big development phase in which no releases are made and we all wait 2-3 years while following the weekly Devblog. (Although since gaining it's nickname, it's emerged that we'll still get bug fix releases even during the Big Wait).
In theory the last Big Wait (though it wasn't nicknamed back then) was the 2012-2014 Living world update.
Development plans for the Big Wait are a rewrite of the map (makes sites more flexible, recodes some nasty buggy stuff from Toady's youth, makes the underground actually work properly and allows for big map changes during worldgen and during play (Cataclysm)). A myth generator (creates the gods, creatures, their ethics, beliefs, magical powers, afterlives and so on to make each new world created more unique - with sliders to fine tune just how unique/dangerous/fantastical you want your world). A set world editor (for creating worlds with your very own specific historical characters, artifacts, mythology and landscapes).
And a bunch more, much of it relating to the procedurally generated magic systems).
Expect more than a few Big releases until the next Big Wait occurs.
(And as mentioned above, the wait for the Big Wait is ongoing and likely at least a year, probably more away). Steam, Villains part 2 and Improved Sieges/Adventurer medical additions to add before we get there.
Development plans (some of them anyway):
http://www.bay12games.com/dwarves/dev.htmlMythgen demo:
https://youtu.be/v8zwPdPvN10(Tarn starts around 10 minute mark - From back in 2016, and, oh look, he's there with Tanya Short of Kitfox games
).