AnotherDayAnotherDollar
Banned
As I understand it, Fox owns Negasonic and just needed Marvel's permission to change her power. So it's similarly a half step. Marvel Studios flat out saying no would meant no Ego and a potentially strained relationship. Had Marvel argued to get Doom for changing Negasonic's powers, I can't see anyone taking that deal. Fox probably would've used another character instead of Negasonic.
As per James Gunn's comments I posted above, Marvel co-owned Ego, so that means they could have still use Ego (and Fox could have used him in the future). They should have tried to co-own or fully own a character they didn't before like Galactus or SS.