With it being in Cardiff, it makes sense to have some guests from programmes filmed locally - i.e Dr Who, Torchwood, Merlin, Being Human, Gavin & Stacey etc.
Personally, I think it would be great to get Danny John Jules & Craig Charles as my son really wants to meet them.
For me, anyone from Charmed, The Walking Dead, Once Upon A Time, Grimm, Harry Potter etc would be great.