The assumption that I'm working on is that you want a sauce that has the body of a reduction but with the eye popping brightness of the juice (knowing what kind of fruit juice would help here). I would also assume that you want the fruit flavor to be at the forefront and not just an accent.
Gelatin is a good idea for texture but might be a problem for flavor. Commercial gelatin is primarily a pork product and if cooked too intensely or too long will taste of pork, or at least "meaty." This may or may not be an issue for you. If its not (like you were making a blueberry sauce to go with duck) than I might suggest using a very reduced meat stock (past the demi glace stage) to thicken the juice.
However, if you want a sharp, tangy sauce (without that "meat" quality) I would start by reducing some of the juice down to a thick syrup, then adding fresh juice while the sauce is at a very low temp. With a little adjustment you should be able to find the balance between a developed, intense flavor with something fresh and vibrant.