In general, IMO, there are two kinds of ideas that are half built.
When you first get an idea for a card, you either have:
1) An idea for a mechanic.
2) An idea for a theme.
IMO, these are the kinds of ideas that could go into this section.
So what exactly do I mean by them?
Mechanic
When you have an idea for a mechanic, you think of a special ability that you think would be cool to have in game.
Most of the time these will be abilities. My personal examples of mechanic based ideas are things like Magnet or Acceleration.
In general you have trouble with mechanic based ideas because you can't think of a good way to implement them.
Questions to ask and that should be answered are:
What element(s) should it belong to?
Should it be a creature/permanent/spell?
What should it be called?
What should the ability be called?
What would the art look like?
Theme
Theme-based cards are different. Usually you get theme based cards when you think of some kind of real/mythical creature or object that you would like to see in the game.
In general for theme-based ideas you already have a good idea of what element the card is from. There is usually only one main thing a theme-based card needs, and that's a good mechanic that goes well with the theme.
In short, if you thought of a mechanic for a card, you need to generate a good theme to go with it, while if you thought of a theme for a card, you need a mechanic that goes well with it.
This section should, IMO, focus primarily on developing these half-ideas.
Anything that has a theme and a mechanic already is ready for the Smithy and to be subject to balancing. (Cost, stats, etc.)