The Eight Legs would be high on my list of baddies. With todays tech I think they could be realised so well, and they were brilliantly understated baddies.
In terms of people, I would quite like to see Rita again from The God Complex. She was a great character. Dead, but great.
I would love to see the Eight Legs return but I think the audience today would expect more than slightly large spiders. No disrespect to them, of course, I just think that these days the fans expect bigger and badder rather than simple and effective. Maybe I am underestimating our modern audience, but that's just my thought.
And yes to Rita. She was awesome.