I really like Spike as a character, but I agree that he is very difficult to write for because he doesn't talk. In a conversation, he usually can't contribute much more than an occasional affirmative or disagreeing grunt or gesture. However, he still manages to maintain a significant role in virtually all of the movies (I thought that he was somewhat downplayed in LBT XIII, possibly because he had to share the screen with another mute characteróthe yellow belly Foobie), and I think it is by observing these that we can best come up with ways to include him in fanfics.
One thing I've noticed about Spike in the LBT movies is that he is often used as a "plot device" character, playing a brief but pivotal role at certain important turning points of the story. For instance, in LBT V, he was the first member of the gang to pick up the scent of the Big Water, and after they got there, he was the one who first spotted the island (and later confirmed the edibility of the native vegetation
). And in addition to deciding for the rest of the gang to follow Ali in IV, he was the first to approach and befriend Tickles and find the night flowers, and was responsible for the "defeat" of Ichy and Dil, by knocking the former into the latter's mouth. Giving Spike small but significant parts like these would probably be a good way to increase his presence in a story.
Spike also has a number of individual traits and abilities that can potentially be utilized by fanfiction writers, but often seem to be strikingly underused. After Chomper, Spike appears to have the best sense of smell among the characters, and seems to be quite observant. (As my examples from the previous paragraph demonstrate, he is often the first to notice and/or investigate things.) Also, while he appears fat and gluttonous, Spike has shown himself to be one of the strongest members of the gang, probably second only to Cera. (Notice that whenever the characters stack up to form a living tower, Spike is usually the one holding up everyone else.)
Finally, the TV episode "Through the Eyes of a Spiketail" is worth noting here, as it offered us an insight into how Spike's views the world. I personally quite liked how this was done (with the exception of Spike hearing the dialogue of the other characters as grunts and squawks similar to his own), and while others may disagree with me, I think that some of what was shown of Spike's perspective in this episode could probably be applied to his behavior in fanfics.
