The characters from TMI absolutely show up in the Dark Artifices. Here’s a snippet from Lady Midnight featuring them.
The Dark Artifices series takes place five years after the events of The Mortal Instruments. At that point Clary and co. will be in their early twenties so it will will get to check in to see where they are in their lives, what jobs they have, who has married and who split up, etc. I can promise at least one wedding. 🙂 Clary and Jace and their friends are part of the fabric of the Shadowhunter world, so we will always be able to check back with them, no matter who is telling the story.
(For more detail about them, see Tales from the Shadowhunter Academy.)