I even forgot to ask - is there a league mechanic this league?

When we go through campaign, whats the actual league mechanic?

There was wisps, temple altars, what about now?

some crafting mechanic. kinda meh

Runes of Aldur is the actual league

You'll find an area every zone where you can choose from a list of rewards then you'll fight monsters to claim that reward.

If you've played poe1, the mechanic is kind of like beastcrafting but found within a map, and without hunting all the beasts.

-Find Rune pillar thingy in zones
-Interacting with thingy shows list of recipes using runes as ingredients
-Pick a recipe to start a fight, win the fight get the reward recipes reward.
-bigger recipes have bigger rewards and harder fights

I'm still not sure how independant the Runes of Alduur mechanic is from Expedition or if the new Expidition is entirely within the new league.

Nased on the removal of all the npxc I think it is safe to assume the league mechanic is somehow part of new expedition that will come out at a later time.