In the interest of efficiency and performance, the stated goals of the GameDataService's design, please allow the use of aggregate functions and field selection in the queryItems API.
Both features allow developers to reduce the amount of data that needs to transfer between data service and cloud code.
Alternately, allow us to fall back to MongoDB/collections for the special cases where that is the optimal operation. (This assumes that MongoDB and GameDataService use the same underlying architecture.)
We'll pass your feedback onto the product team for review.