OK, I can't believe I haven't done this yet!
Annie already specified Treguard, Lord Fear and Lissard... so that saves me having to use one of my spaces for Lissard. Five more I'd like to see back are:
Motley - always my favourite character. He's funnier than Folly (and the acting's a bit better), and I wasn't keen on Sylvester the Jester - I've no idea what they were trying to do with that character. Paul Valentine's a great actor and I loved him as Sly Hands, but if I had to choose, I'd say Motley.
Greystagg - as has been said before. I like her neutral stance and the fact that she could aid Dungeoneers, but be almost deadly too (although she never killed anyone, so...). She was played well, and there's much more they could have done with her.
Hordriss - although I preferred his more neutral stance from series 4 ('bad' Hordriss in series 3 didn't work for me and 'pure' Hordriss in series 8 also didn't!). We developed, once, an updated version of Hordriss for Knightmare RPG - a return to his unaligned stance, but he still rewarded winning teams with trophies, as it was such a rare occurrence. Something like that would be good.
Someone Dangerous - there needs to be, somewhere, a constant threat: a genuinely dangerous character who could and would kill a Dungeoneer. I don't like Skarkill very much, so who else? Snapper-Jack? Raptor? Cedric? Automatum? Personally, I'd invent someone new to do this, but I'm sure there's somebody threatening enough to be so.
Daisy - I'm of the opinion, really, that Treguard needs an assistant. Both Pickle and Majida have had their time, and for what it's worth, I liked Daisy!
Other characters I'd like to see come back include Smirkenorff, Hands, Merlin, Theodora, Greystagg, Bartram, the Grimwolds, Garstang, and Stiletta!
I'm very tempted, obviously, to include Robin as well...
How might you change Knightmare if a new series were to be made?