Out of all the outfits in KP, I personally have to admit that the Resistance had the best outfits. They so should of recylced the "Future Gear" for Kim in Season 4, but then again, that's me being a Resistance fanboy
The Kim Possible: ASIT movie. I so see this happening, but since joss wasn't intorduce into the series till after that movie. well; you know, but she is definitely Kim's replacement if something did ever happen to Kim in the series at least. In the fanfic world she's easily over looked cause of her decision towards the end of the episode she appears in of fanning from Kim to Ron.
I agree with you that Joss wasn't in ASiT because TPTB hadn't decided to come up with her yet.
Overlooked in fanfics? There's a number of stories on fanfiction.net about her (cpneb's somewhat famous for promoting the "Jade" ship (Joss/Wade), and several others have followed suit).) I'm fine with Jade, but I do prefer her 'shipped with someone else (you can probably guess who), and that could make for some interesting stories...
I know what yah mean. I seen mention of it Fanfiction.net myself. The your going with it is your thing. The Joss idea could keep the possible series going if they really wanted to have someone take over for kim when she hit college and settling down. Unfortunately neither of those will happen now that they filled as many of the wholes up as they could.