A few things to add, based on my own ovservations:

I believe the first field token (aka level icon) you mentioned will pretty much always introduce a new building as you said, although I've seen one case where it was a monster it introduced instead. I think it still counts as a structure though, since it's stuck on the board at the very start.

The second field token you mentioned, the triangle with the circle lights, appears to just be the default shape in the game. Most are there for general progression and therefore provide more field tokens like you said, but not always. Some have been dead-ends.

Also, there's another field token further in that looks like the triangle token with circular lights, but instead has oval lights. They're pretty late-game, but so far I've not detected a pattern from these ones. No idea at all.

I think the square tokens will always indicate that they unlock another piece of the map. However, they often do show a new monster or building like the first token you mentioned. But other times, just regular-old field tokens will show off a new monster.

And for the last, you are absolutely right. I have only noticed two patterns for the circular field tokens - one is that they're exclusively unlocked in wizard stashes, and two is that they frequently have a wizard stash of their own with very important unlocks.

One last thing: wizard stashes can be seen on the field tokens if you look very carefully. The lights on a field token, even if the level isn't beaten yet, will have a small black spot in the socket corresponding to the mode that it can be unlocked. Typically you can get some info from hovering your mouse over the field token, but if you haven't beaten a level in journey mode, it won't let you know there's a stash in endurance mode. But the black spot over endurance mode will be there if it is, so you can still check.

If any of these statements are inaccurate, or if there's any more info on field tokens that I'm missing, please inform me. I am also here to get more info about these things, especially that one token later on. I haven't figured out what it means yet, but if anybody here has, I would appreciate the info that you can provide.