Thinking it over, I've come up with some more suggestions.
It might be worthwhile checking out fanfiction.net to see what other folk are up to, what sort of ideas they have and how they set them out.
My second suggestion is to go have a look at the work of my favourite fanfic author, "Netraptor". Her website is:
http://www.netraptor.org/fanfiction/viewuser.php?uid=2The reasons I like her work are:
+ Plotlines are interesting and hang together well
+ The focus is on Sonic, Tails & Knuckles, rather than every single Freedom Fighter ever introduced
+ Although there is a self-insert character (see below), she doesn't act *too* self-inserty and indeed takes more of a back seat as the series continues
+ She's done adaptations of the video games (S3&K, SA, SA2, Sonic Heroes, Sonic R, Sonic CD...)
+ Grammar and spelling are both pretty good, with few exceptions
+ She doesn't tend to use deus ex machina - i.e. she doesn't have some entirely unbelievable thing happen in order to pull Sonic et al's butts out of the frying pan
+ I feel she has a real handle on how the characters work - Sonic's not simply the blue blur, Tails gets more independent, Knux clearly has a great love for his island and the Master Emerald. As for Metal Sonic, she made me really empathise with his character, which is pretty unusual for me!
+ Although the characters face trials and hardship and mature etc., she doesn't overload the angst
Downsides to her work for me are:
+ Religion tended to be quite blatant in the early part of the series - as an atheist, that began to annoy me slightly
+ The author's self-insert character, Slasher, tends to save the day a little too much at the beginning of the series. As is usual with a self-insert, she can fly further than Tails, is stronger than Knux and can keep up with (or even beat) Sonic! Oh, and she's a better strategist than Sally...
+ She had the tendency at first to introduce various new fan-written characters (either her own or, with permission, other people's). This meant that there was a rather large supporting cast, possibly too large. Again, these characters were used less as the series went on and the focus was back on the main characters
Anyhow, I hope this helps. Obviously these are just my opinions and there are loads of people out there who will be looking for other stuff!
DW
EDIT: Forgot to put in the URL, d'oh!