To me, the element of fire can be represented in aikido by techniques such as the first five techniques of junana hon kata, or randori no kata (or "the 17" as we often call it).
Like these, fire is direct, right to the center line (as opposed to working from uke's elbow or wrist). It often moves in short, straight lines.
With fire, one's ki moves quickly, instantly lighting up the sky like lightning, and fades just as fast.
Fire is definitely not soft. Rather, fire is sharp and capable of taking an opponent down in a single cut. There are no joint locks or pins, no effort to control or suppress; fire simply strikes (hence, ateme waza).
In terms of "go no sen, sen no sen, and sensen no sen", fire lays more in the realm of sensen no sen: our opponent has perhaps only the intention, the thought of attacking. Scarcely can he begin his move when the flames sweep in and level him like a forest fire.
Which means fire can also be impetuous, eager, quick to action. When conflict arrises, striking is his first thought. He is motivated by feelings of justice, of delivering what he feels his opponent "deserves." Consequently, fire can easily veer off into self-righteous fervor. It can convince a person that their own path is the correct one.
Fire is like youth: boundless energy mixed with little fear. The line between "brave" and "foolish" is blurred. Beginning students start with fire. It's fast, powerful, easily grasped. Their eagerness to use what they have learned, however, can leave them burned.
Fortunately, they are also quick to get right back up and try again. They are not easily discouraged; and when they are, their resolution is soon redoubled.
You see, to my way of thinking, there is a yin and yang to each element (at the risk of mixing my metaphors). There are advantages and drawbacks, strengths and weakness. Both sides of the coin much be understood and respected.