Deceptively Destructive Theme
Niki - Family Portrait (Pink) - good but hard song to sing - it doesn't bother me, but a lot of people are put off Niki because of her story, so this song wouldn't help matters

SD: Was going to suggest "I Begin To Wonder" but that's already been done, so what about "Doin' It" (Liberty X) - as opposed to "Holding On For You" which would be erfect - certain melancholy songs might work for them as some fans have mentioned wanting to see a more serious side to them, but I can't see how anything remotely incestuous would go down well - least not with SD themselves!
Anyway, what do you think?
Leon - Gay Bar (Electric 6) - Not saying they're the only demographic he appeals to, but his female fans wouldn't be too impressd, and I'm not sure how he'd handle the performance side of it - even the vocals are basically just shouting, but I doubt that'd cross most people's minds before they heard it.
Rhydian - This is hard! I know what'd put me off ("Chasing Cars" or anything to do with his mentor!) but I doubt that'd be so universally offputting

I was considering one of the "standards" Simon wheels out every year, but they don't prevent acts from doing well - so maybe something with a sentiment like "I've already won this" - "We Are The Champions" maybe, although as he'd nail that so well, he'd have to go overboard in his VT explaining why he was singing it!

(Another "advantage" of that choice would be how same-y it would look after he (along with 2 other acts) sang Queen last week!)