If you could add an 18th type to the type listings what would it be and what would its strengths/weaknesses be? Would any previous Pokemon become this type? Would it be defensive/offensive/physical/special/fast ect.?

I think the better question is “would I do it at all”?

There are a lot of things I’d change about Pokémon, given the chance, including rebalancing the weaknesses and resistances of the existing types, but I don’t really see the need for a completely new one; I struggle to think of any concept that couldn’t be adequately expressed by one of the existing seventeen, or a combination.  Moreover, when Dark and Steel were added, there were only 151 existing Pokémon – now there are 649.  I wouldn’t want to add a new element without also committing myself to creating upwards of 35 Pokémon with that element in a single generation, and probably adding it to at least a dozen existing Pokémon.  From a purely practical standpoint, that would be a nightmare.

Still, you did ask…

After Dark was added, a lot of people started clamouring for a ‘Light’ or ‘Holy’ type, and I suppose I could go along with that. If so, I would want to use it as a balancing mechanism, so resistance to Dragon attacks would definitely be on the cards, as well as weaknesses to Poison and possibly Psychic attacks.  Resistance to Ghost attacks is sort of necessary for thematic reasons, but Ghost is already one of the weaker offensive types, so I’d really have to give Ghost an extra strength or two somewhere else to make up for it.  I think I would make Light and Dark super-effective against each other.  As for old Pokémon with this type… well, Volbeat and Illumise would be good candidates for Bug/Light, but no-one cares about Volbeat and Illumise.  Cresselia would definitely get it.  You could probably get away with slapping it on Clefable and maybe Arcanine.  Togekiss would probably get this in place of Normal.  It could potentially replace either of Jirachi’s types.  Sunflora would be an obvious candidate.  I’m really tempted by Dark/Light for Absol, but I’m not sure you could get away with it; maybe for an evolution.    Electric/Light might work for Luxray.  I can’t really think of any fifth-generation Pokémon it would work for, aside from maybe Cryogonal, at a stretch.

I will emphasise again that I would actually prefer not to add a new element, but if Game Freak showed up on my doorstep and told me “we want you to be team leader on the next Pokémon game, on the condition that you let us have an eighteenth type,” that’s more or less how I’d handle it.