Regarding adding the trait slot(s) as a race/faction ability: if it matters to you, having extra selectable trait slots will probably not be a very beneficial ability for the faction when controlled by AI. Just look at how frequently the AI chooses the "base" unit designs without even using all three default slots.
At the very least you'd need to pre-design units to make use of all the slots at various tech levels, and I don't remember anyone ever having tested whether or not the AI will choose designs with > 4 traits.
For larger values of additional trait slots, it could potentially also cause problems on higher difficulty levels where the AI gets "bonus" traits for its units applied on top of the ones in the unit design. Worth testing, anyway.