okok...
problem : anything you give to federation, you are giving to vassals\overlords. as they are in no way shape or form excluded from federation , you need to work a new way to limit one of the 2, or renounce .
the solutions are varius , like making the " vassals join federation " have bigger consequenses for the federation cohesion , or create a new federation that doesn't allow overlords\vassals , and that has a new more deep relation with members and alow for "federation holdings" .
what i hope to see for federation is a bigger indept of theyr laws, in particolar , with the rework of the situations , there can be new laws that could be extremly interesting. ( for example, a laws like " energy cooperation " that could give 1% energy production bonus for member , but if one of the member enter a deficit of energy the federation will have a situation that will tick towards bad ending , and there could be votes for how to manage the sistuation . )
i'd like for federation to actualy have a repercursion even in internal politics , if some laws are passed.
and there the holdings could be key for that. like a "federation capital " etc.
but , it need to considerate that overlords\vassals can ( and should probably aim for ) be a federation . this would mean having the overlord holdings , the federation holdings , all the advantage of the vassals specialization and the federation perks .
btw, it would be cool to have the possibility to have contracts between federation members too , like giving tech in exchange for basic resources ... actualy, that would be actualy cool to change the " diplomatic exchange" that there is at the moment in the game. i would totaly see an empire "pay" 20% basic resources to have a defensive pact.
edit: immagine a megacorporation selling energy to half the galaxy in exchange for treaties or research\resources