(warning, not proofread)
Hi there! This will be a bit long, so I'll bold the relevant sections for people who don't want to read all of it. ;). Following DK's comments, yes, I did find may way here via a piece of GWing/Trisphee fanfiction. Let me tell you about that. :)
I did bump into DK's fanfiction "Onward to Trisphee" at ff.net and my first reaction was "what the hell is Trisphee?" I looked like a standard characters-in-hi-school fic, which are common enough. It showed the anime characters encountering this avatar site called Trisphee. If all it was was a high school character fic, I might have stopped there, but why I think it worked was because I learned through the fic what Trisphee was, what an avatar site was (I had never even heard of them before then), and what sort of stuff happened there (the fic took place around the Taskal War event). I had been on forums before and since the forum I usually on was changing for the worse, I followed the little link DK posted and ended up here. (I know at least two other people followed as well but I think I'm the only one that stuck around).
On the technical,business end of things, I think it's been established that Tris does not have the resources to devote to an official webcomic for the site right now, but to elaborate a little bit, I think staff right now are taking the safe route as far as site development. There is always something to be said for advertising, like DK's fanfiction piece, and I think a web/fancomic would be great for that. But, to have a regular comic going on tris would need more resources, which means we need a larger user base than we currently have. Staff could put resources towards the webcomic and hope that it draws the user base in to pay for itself, but it's a risk. If it doesn't work, it becomes a waste of resources the site may not have. On the other hand, a user fancomic gets advertising, without draining the site of needed resources.
Okay, that said, I like the idea of a fancomic. :)
Apart from the advertising bit, which is a good bonus, I think it would be fun and entertaining to read. Not sure how easy they are to find now, but in the past a few people did little strip comics and pictures depicting NPC's in events from the trisphee story as well as just hypothetical scenarios. They were cute, and helped endear people to the characters.
On the story: it's fine I think to go along with the tris story, if that's the plan, but I also think it would be fun to make little short strip comics that just show an event or two, or maybe something that has even yet to happen or happened "off screen" so to speak. Since it's a fancomic, there is a lot more freedom to work with the characters, though I would like to see them acting in the way they normally would. Little short vignettes would allow for a break from a longer story for bother the writers and the readers, and provide more ways for potential users to get to know the characters. It also allowed for multiple writers to do their own thing now and then, especially if working together on the one big project starts to get on everyone's nerves (and it likely will at some point).
On the art: it would definitely be a lot of work for one or even two artists, and like DK said, if needed, people can decide and share a style, but I personally think that multiple artists or different styles isn't a bad thing. Consider something like Animatrix or the Batman movie-thing that showed multiple Batman stories but all done in different animation styles. I found them very interesting to watch. The stories were related as in from the same setting, but since each were contained into themselves, it wasn't a shock to see the different art styles. I think the same could be done for a fancomic. Strip comics would allow for a free mix of styles, and even in the larger story, as long as there's a chapter break in between, I think switching styles/artists now and then wouldn't be a bad thing, especially if it's mentioned before hand that that's what you guy will be doing.
I can't devote much time to a project like this, but if you do go the strip and full story route, it wouldn't surprise me if other people decide to little things now and then. I'll read it if nothing else. :)