One thing you could do for this kind of thing is to have the subplot be an indicator for the main plot. The example I'm thinking about is Inside Out: the "real" story is Joy and Sadness trying to get back to Emotional HQ, but there are two subplots. Fear, Anger, and Disgust fight and bicker amongst themselves, demonstrating how poorly they work without Joy and Sadness, and Riley struggles with her new life in California, demonstrating how badly she needs Joy and Sadness to get back. If we want to talk in Dramatica terms, the Fear/Anger/Disgust subplot would be the Forewarnings (or maybe Prerequisites), and Riley's would be the Consequence.
So for example, you might have a subplot in a superhero movie where the main hero's boyfriend takes care of a gaggle of children inside the villain's underwater base, and as the air runs out, he struggles to make sure the children stay safe and healthy. This reminds us of the Limit, the Consequence, a bit of Precondition, and Forewarning. Maybe Cost, too? In any case. This subplot helps to remind us of how important the central quest is and that time/options are of the essence. You could do crops corrupted by dark magic, technology breaking down, a trapped princess listening to the villain's plan... you get the idea.
Another thing you could do with a subplot is set up dramatic irony and/or foreshadowing. Maybe the main story is about an explorer descending into an ancient ruin, and the subplot is about a journalist centuries later revisiting the temple and discovering a dead body. He knows it's one of the members of the explorer's party, but he doesn't know who it is. Or it could be a story where one of the villain's henchmen seems to have lingering doubt about the villain's methods. You could have some dramatic irony when they meet the heroes and the heroes try to kill them--or you could have them leave secret gifts for the heroes, which the heroes eventually find.
...I don't know how useful all of this is. I guess my point is, the subplot doesn't have to be a full-on storyform if you don't want it to be. It could just be a small brush against the story proper. It'd be easier to give advice with a little more info.