Another idea is that a powerful Pokemon villain teams up with one of the Teen Titans' enemies. Ash and Raven are forced to work together because they are the only ones who can sense the true nature of this new threat. They start off not trusting each other, but as they face challenges, they come to respect one another. For example, Raven might be initially skeptical of Ash's ability to handle such a big threat with just his Pokemon, but as she sees him in action, she changes her mind.
Maybe there's a device that mixes the powers of Pokemon and the abilities of the Teen Titans. Ash and Raven find this device and have to figure out how to control it before it falls into the wrong hands. Along the way, they learn about each other's worlds and grow as characters. Ash could teach Raven about the bond between a trainer and Pokemon, and Raven could show Ash some new ways to use his strategic thinking with her knowledge of magic and combat.