What sauce can I just to de-sweeten a cheesecake?

I'd make a simple raspberry coulis by blending fresh raspberries with a little water in a blender, with perhaps a touch of sugar depending on their natural sweetness, then passing the results through a sieve to remove the pips. No need to get fancy, the cheesecake is the main event!

Sour neutralizes sweet and vice versa. So you want to make a sour sauce with almost no sugar.

The obvious idea is to use a fruit based sauce. Citrus goes really well with strawberries and cherries. Lemon, lime or maybe grapefruit juice should work well as base. Mix them with water to the desired acidity and thicken them with pectin or starch. Add zests for aroma. If you want a dairy based sauce, add a fermented product (yogurt, buttermilk or sour cream), milk or cream will curdle.

If you prefer it more exotic, you can make a zabaglione with a very dry wine (leave out most of the sugar).

I don't feel that bitter can mask sweet and vice versa, but many people seem to think so. If you are one of those (e. g. if you sense coffee as less bitter when it has sugar), you can make a bitter sauce. Bitter chocolate, cocoa powder (preferably non dutch) and coffee can be combined with milk or cream to produce a good sauce. If it has to be thickened, use starch again, or add mascarpone.

You can also divert from the sweetness by using an unusual, strong taste. It will dominate the sweet pudding and attract the attention to itself. For that, you could use a hot sauce (e. g. by adding cayenne to either a fruit based or chocolate based sauce). Or you could make a non salted pesto with a fresh herb. A comination of peppermint, almonds, almond oil and some lemon juice could work well, but something more unusual for a sweet dish would be better, like basil. Whatever you choose here, it is very daring, and it is easy to create a combination which won't work. So only try it out if you don't mind a fail or two. Else, stick to the sour or bitter sauces.