1) You can add extra heavy cream or cream cheese to maintain the creamy consistency. Another option is to create a roux (a mixture of butter and flour) to thicken the sauce.
2)Without Parmesan’s sharpness, you might want to add a small splash of lemon juice to balance the richness of the sauce.
3) Parmesan provides a salty, nutty flavor. You can also experiment with nutritional yeast, which has a cheesy flavor and can add depth to the sauce.
4) You can use other cheeses like cream cheese, mozzarella, or even grated Pecorino Romano (which is sharper and saltier). Each will give a different taste and texture, so choose based on preference.