Mixed Heritage: “Your creatures count as both of their fused parents races”
I think this would be an interesting trait, allowing unique builds focusing on two racial synergies.
I think, balance wise, it would be onpar with other avatar traits, as the racial synergy traits don’t generally seem any stronger than non-racial synergy ones (aside from the Hoard health traits being noticeably strong)
It should also be noted, that at the moment, there is little incentive to run a full team of a given race, but rather cherry pick 1 to 3 creatures and then cast the spell to change the rest of the races, or use a cockatrice. Here there would be an incentive to use a full team, on the secondary fusion parent side, as the bonus races would be unaffected by spells and other traits.
N.b. if this were to hard to implement in the game, an interesting trait along similar lines might be:
Species Projector: “All ally creatures also count as of the race of this creatures non-avatar fused parent.” Which again would allow multiple racial synergies at once.