I just commented in the Series 14 speculation thread that I would LOVE to see a Castle themed series.
Yes, I know that Castle is already a viable theme in LEGO sets...BUT...take a look at the last Castle theme releases.
*Yawn*
Boring as heck. 'Kingdoms' was FAR superior, in style AND substance but, even then, we lacked 'MORE'. More lords and ladies, more town folk, more civilian buildings, etc., etc. I don't know about the rest of you but, I'm rather tired of seeing "blacksmith shops" as being practically the only civilian 'buildings' in a Castle theme. So, by doing a themed Castle series of minifigs, we could at least have a correction to ONE of the problems (lack of themed figs NOT in armour, or wearing a crown). Sure, it doesn't do much about the cruddy selection of buildings but, at least in THAT regard, I can build my own.
Can't think of what figs might work well in such a themed series release? Take a walk around a Ren Fest and I'm sure you'll see HUNDREDS of ideas.