I always thought that with mixed races, players should have to choose one trait from each to carry over. You know. players get 2 traits free of their choice. 1 from each parent. like Superior Dark Vision from Drow and maybe Luck from Halfling. No increases or decreases for Abilities.
Unless... If you want more traits, you have to pick a disadvantage trait. Want Keen Senses as well? Gotta take something like Sunlight Sensitivity. Want Brave too? Gotta take maybe a -1 on an Ability score. Something like that.
