1) Julie-Su returns along with Simon and Floren-Ca (her step-parents) in Sonic #98
2) Dimitri becomes Enerjak in Sonic #100
3) Charmy is back in Sonic #93 because Sonic #92 is going to be the Sonic Shuffle adaptation
4) Ken's not sure if Tikal will return
5) Nic and Nack aren't going to be the only ones hunting Knuckles in Sonic #91
6) Tails and Knuckles will be working together during "The Chosen One" storyline (Sonic #95-97).
Here are some direct quotes from Ken Penders that I managed to copy and paste (mostly from the Q&A part):
Sega is going to exert more control in rgards to using the series as much as possible as a promotional vehicle
Sega will have us adapt any SONIC game they put out. That's called marketing, and we can't refuse to
There won't be any Chaotix stories, Blue Raven, but the characters will be making appearances I just can't say when they will do so other than Charmy
[on Knuckles series] That's why the book is no longer published, because even Archie had a difficult time getting the books into the stores
It'll be sometime after #100 before we can address Remington and his secret
I highly doubt Knuckles will be on speaking terms with many people, let alone Sally after SONIC #100
Just to give you some idea of the insanity in producing the latest issues we've been barely getting the books out before deadline due to a lot of miscommunication and other snafus
Geoffrey will become more involved with Sally, for instance
Sally will re-examine her feelings towards Geoffrey
Keep your eyes on Mina, everyone, as her role is going to increase as well
SONIC #100 will involve a crossover with SONIC and KNUCKLES
Xenin could return, Mariah, but I'm not sure when, because I'm more concerned with Enerjak and the events leading up to SONIC #100.
Manny is no longer associated with the books in any way. He's too busy working for Nickelodeon
[on Lara-Su] I haven't worked out her color scheme yet, although I know I'll have to real soon as she will be appearing sometime between SONIC #100 and #112
Something does happen to one of Sonic's parents by #100
We will be addressing whatever happened to the Overlanders
You will meet the Ultimate Villain in SONIC #100
I don't think we'll see much of Remington or Komi-ko beyond a token appearance, unless...
My edit: the chat moved too fast for me to copy the rest of the statement, but Ken and Justin have proposed a mini-series to the publishers and they're waiting for approval or disapproval
If you missed the chat, there's most of the relevant Sonic/Knuckles info.