I have been wishing for a NUMB3RS crossover for years now. The main reason is that they are both FBI, but more than that, I want to see Charlie work with Reid because they're both math geniuses. It would take place wherever NUMB3RS takes place, which I think is Los Angeles, just because I figure the CM team travels while the NUMB3RS team is more static. Crime would be a murder, or course, or more likely serial murders because that's what the CM team normally investigates. I would have Hotch work with Don and Reid work with Charlie; I'm not familiar enough with the rest of the NUMB3RS team (I actually don't watch NUMB3RS all that much!).

I'd also love for an NCIS crossover in order to get Garcia and Abby together. I think Garcia would also work with McGee. Gibbs would work with Hotch, and I'd put Reid with Ziva, Morgan with Tony, and I don't know what I'd do with poor JJ. It would have to be a serial case involving the Navy or Marines.