Had a bit of a look around and can't see any mentions, has anyone done a mod or planning a mod to introduce snakes (normal or giant) into Skyrim, could make for some good wilderness encounters or even dungeon traps (snake in the face)?

Well, snakes are cold-blooded creatures and since Skyrim has a cold climate, it's somewhat apparent why snakes aren't featured in Skyrim. As for a mod, I've found nothing of the sort.

The lore would support them being there - Ralof makes a snide remark about giant snakes when you're escaping Helgen with him and the spiders attack.