What's a good replacement for cream cheese in icing?

You can substitute Mascarpone cheese.

Cream cheese has about 33% fat. So look for a plain cheese with similar fat and (for cakes) no herb or garlic additions. Examples: neufchatel, mascarpone, cottage cheese. In a pinch, use soft tofu. For that or cheese with lower than 33% fat, bolster the mix with some butter.