Do sights count as modifications? Because I swapped the stock plastic sights on my Gen 4 G-19 for a set of Warren Tactical Sevigny carry sights, tritium illuminated front dot with plain rear. I know, it sounds weird, but I really like having a dot on just the front sight. The front sight stands out NOW when you go looking for it in a hurry, and even in low light having only one dot isn't problematic for me. If the front sight is to the left or right of the rear, I can definitely feel that in my grip.
Otherwise, bone stock inside. Just add ammo and go.