Dexter's Laboratory: Ego Trip (1999)

Dexter's Laboratory: Ego Trip is a animated film based on the American animated television series Dexter's Laboratory. It was produced by Hanna-Barbera (now Cartoon Network Studios) for Cartoon Network and originally aired on December 10, 1999. The special follows Dexter, the series' protagonist, as he travels forward through time and meets futuristic versions of himself and his rival Mandark.

More at Wikipedia.