*this post likely to be edited with an actual list when I'm not at work*
Not a lot of dragons, actually. A dragon or two is good, but you can always use a different color as a stand in.
That isn't as true for some of the other suggestions.
For example:
The tower and catapult would be great. I'd make the tower stackable too, so you could have a taller one. If done well, this would likely be one of the biggest sellers in the pack. Or not. Maybe other people don't use towers and castles all that much.
Ideas I love, but may not be as popular:
I love the Juiblex idea. I'd buy a pack just for that one. As a matter of fact, I'd buy lots of slimes, puddings, and other things like that, but I'm thinking not many would agree with that.
Along with the tower, a gate would be great. It could double as a door. Heck, I'd do walls and other terrain too, but I don't think WotC will just yet.