Alright, I've been having several thoughts for some time now.  1) Normal/Dark . Sure it has a 4x weakness to fighting, but offensively it can be a huge threat to even the best walls out there. It could be a gorilla. 
2) Fire/Water . I know we got Volcanion, but I don't think of it as a legit mon yet. I was thinking of a dolphin or a narwal that sprouts fire and has fire-like marks on it's body, like Heatmor. Another thought would be an evolution of Octillery.  
3) Fire/Grass . Who doesn't want an fire-breathing green & orange Echidna with a tail that resembles a young plant? That thing could be a wall or a sweeper. 
4) Fire/Poison . Weezing's next evolution (with levitate or unaware) or a vicious red panda. 
5) Electric/Fighting and Electric/Dark . Introducing two rival pokemon that represent Order and Chaos respectively. The fighting one would be a bulky humanoid and the dark one would resemble more of a beast like Nidoking, just thinner. 
6) Ice/Fairy . A frost pixie that hates steel and technology.  
7) Poison/Psychic . Everyone knows that psychic mons are the most intelligent. This one has two natures clashing with each other: high spirit and raw beastly power. Thus, it has elevated HP, Def, SpA but lack speed. To picture it, imagine a cute tarsier smiling, with upper fangs coming out of its mouth and three elongated claws in each arm. 
8) Rock/Ghost . This one could be a legendary mon, bulky as no other. Ideas: A totem with spectre arms and tail or a gargoyle  What do you think?