- Pikachu & Mimikyu
- Rayquaza & Milotic
- Seviper & Arbok
- Kabutops & Aerodactyl
- Mega Steelix & Mega Gyarados
- Haunter & Mimikyu
Oh, and if a Pikachu & Seviper card would be released I’d be all over it. 
Here a similar earlier thread. Some of the suggestions there were pretty good as well, like Magikarp & Feebas; Metapod & Kakuna; Alakazam eating Vanillite with its spoon 
; Arbok & Weezing; etc.
Greetz,
Quuador