From what we know from stream bits and teasers (all non-final):
- Founding of a Federation is unlocked by a Diplomacy Tradition, making it mandatory pick for someone going for Federation play (unknown whether same unlock is required to JOIN a Federation).
- The Diplomacy Tradition has an upgrade reducing Influence Cost for diplomatic agreements (including Federations)
- The Diplomacy Tradition has a few unique bonus' to being in a Federation (I think it had a +% Unity generation if in federation and something with naval limits)
- Presumably the AI logic for joining federations remains unchanged (and there is no hard limits on who can join what, for players)
Additional stuff:
Having Foreing Ties (I asume that incldues being Vassal, Defense Pact and Federation) increases Xenophile Atraction. There will also be Ethos atraction if your Ethics varry, wich a Pacifist could actually use for the Archeivement:
https://forum.paradoxplaza.com/forum/index.php?threads/1-5-and-the-insidious-pacifist.1004309/
Is it through the diplomacy tradition only or is it a tech that can still be researched? Will it still be possible to federate a xenophobe and and a xenophile? Will the traits and ethos need to match?
Afaik no longer a tech. Only the Tradition. Note that it also gives more Diplomatic Relations.
Xenophobe and Xenophile could Federate. Xenophile and Xenophiba pretty much counter one another. However the AI personality might have considerable effect here, as most Xenophobe AI personalities have acceptance penalties for Defensive Pacts and Federations.
The Xenophobe has to consider that just being in a Fedeartion likely drops his Xenophobe atraction and gives him Xenophile one. Until that takes effect, he will propably be a blocker to Federation Diploamcy - Xenophobia means he hates everyone, and thus deny admission of other Empires.
Ethics already need to match largely, But again, the AI Type tends to be the bigger Factor.
But regardless of relative realtionships, unless they are Metalheads of Fanatic Purifiers, they will form Federations with enough shared treat.