Okay, creating a big plot is easy. You define a conflict big enough to get everyone involved, and then just figure out how everyone fits into it. Subplots, however, are harder.
A subplot doesn’t usually involve more than two or three characters; just because a number of characters may know of it, doesn’t mean that they are involved. However, let’s cover ourselves by pointing that there are numerous kinds of subplots, and one is the plot’s subplot (yeah, the plot can have its own subplots); however, I’m going to leave that one alone for a little bit, okay? The major kinds of subplots are side-trip subplots, conflict resolution, romantic subplots, and running gags.
Side-trip subplots are usually only seen in long-running comics. The basic idea is that occasionally you need a break from the plot, and so one or more of the characters are sent on a personal quest, even if that personal quest is just to get a great tan. This gives everyone a chance to blow off some steam, and just try something new. In regular comics, they would be considered one-shots and limited series; in webcomics, they are considered “intermissions”. Nip and Tuck has some great examples, as do most of the better comic books. If you want to see a great quest subplot, track down X-Men (old series) #247, where the X-guys go and try to get drunk in Australia, but end up saving the world from an invasion (they were making fun of DC’s Invasion maxi-series, which needed to be made fun of). Other examples of quest subplots are from TV shows; you know, the episodes which have little to do with the regular plot, but nonetheless showcase one character and is usually done either more dramatically than the rest of the series, or, more likely, more humorously? Yeah, that one. They’re great to do when it’s getting too serious or too silly, and you need a break. Just mentioning it, as it’s done so rarely, but making sure that you don’t trip over it when it does show up.
Conflict resolution is when the character is allowed to resolve his personal conflict, but in such a way that it doesn’t affect the plot directly. These can be highly useful when you want to highlight just why someone is part of the plot, or what they offer to the team, but can’t figure out how to make them part of the plot proper. As long as they involve the theme of the plot, they should be okay. If, on the other hand, you just want to have fun with the character, wait until you hit a point where the comic needs to take a break from the main action, and involve the character in a side-trip.
You should always have at least one romantic subplot. Obviously, a romantic subplot involves at least two of the characters, and may involve more of them (such as the dreaded romantic triangles). The basic concept is that they tend to draw in readers by promising sex, even if does just tease it, and, well, is arguably realistic, as it’s just a matter of time before someone would fall in love. In shojo manga, they are pretty much mandatory (girls love romance, and it’s part of the wish fulfillment represented by such characters); in shonen manga, however, the romantic subplot may either be severely downplayed, or be replaced by a relationship that’s based on hero-worship, or may even feature a same-sex relationship. Just have fun with it; even if it degenerates into a love cube, they tend to add to conflicts, as well as make for some interesting denouements.
Running gags are a very weird kind of subplot. Some actually involve plotting, others are just interesting scenes strung together. Some are even just quirky observations. Nonetheless, they do need to be noted. The best kind of running gags are when you use them to break up the pace a bit, and put them towards the front, so that they add to character development. Others can add that special “UMPH!” to the finale, as a character that has, for example, been trying to do something all during the plot, keeps getting frustrated because he just about but not quite nails it, and then manages to do it at the end. These are usually sex-based (such as in “Porky’s”, where Peewee is doing his level best to lose his virginity, but fails horribly several times, until he loses it in the movie’s final scenes), but can be tied to the plot as well (the major invention that the mad scientist keeps getting wrong gives the good guys a major advantage when he finally gets it right), or can be a source of frustration for the readers (for example, having a character’s major secret not reveal itself, but nonetheless constantly being brought up; of course, revealing at the end makes the reveal so much sweeter!). Just remember to have fun with it, and remember to not let it get in the way of the real plot.
Hopefully, that gives you some other stuff to use when you plot your comic. If not, why not? Yeesh…