I always wanted to read a Dresden Files/Worm crossover, maybe one where Dresden and Taylor team up and go first after strong baddies from Worm (say, the Slaughterhouse Nine). Thomas and Molly could come along as well, maybe a Knight of the Cross too (which one heavily depends on what period of Dresden Files we're talking about, but the best bet would probably be Sanya), Mouse certainly should come too. Maybe even Murphy, although she probably wouldn't be able to help much. So we'll have Dresden and 3-5 supporting characters. It'll be very interesting to see how the PRT classifies all this guys, and what the capes of Brockton Bay in general make of them.
The S-9 could try to recruit someone from the Dresdenverse, for some additional crossover fun. Mavra comes to mind, we didn't see her a long time and she could conceivably fit right in. Or someone from the Black Council. Cowl? A revived Corpsetaker? The possibilities are endless.
Then, maybe with the rest of the Undersiders helping, they'll go after strong Dresden baddies (most interesting would probably be the Denarians).
Maybe later some of the S-9 can get their hands on silver coins of their own! That would be a sight to behold.
The story could culminate in an Endbringer crossing over to the Dresdenverse, and the Wormverse capes teaming up with half the supernatural world of the Dresdenverse in order to bring it down.
Imagine the Wardens en masse fighting alongside both Faerie courts, Strength of a River In His Shoulders showing up, even a freaking Naagloshii suddenly swooping in to fight the threat to its territory!
Maybe it could culminate in the Endbringer being imprisoned in Demonreach (maybe thanks to a slight, merely implied nudge from Uriel), after a long, desperate fight. Not before Chicago being half ruined, of course, and maybe some staple characters from Dresdenverse dying.
Jeez, I basically written the thing already.
God, I want to read this fic so much! Anyone up for writing something like this, or knows one like this that is capital G Good?
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EDIT: Since I thought this up there's been a flood of ideas filling my brain.
Dresden's proximity would wreak havoc on any tinker he fought beside (or against if ot comes to it). There could be interesting interactions based on that, in combat and out.
I'd definitely want to see the Travellers + Echidna in this fic. Imagine a squad of evil twisted Dresden clones making their way to the Carpenter's house, gunning to kill everyone there including Maggie!
Marcone, recognizing an amazing opportunity when he sees one, may want a piece of the action in post-Leviathan Brockton Bay, and will have to fight Coil/the Undersiders (depending on time period) to get it.
Dresden opens up the Sight to view one of the capes, and can see the Shard with his own eyes.
If Murphy is involved and crosses over to Wormverse, it's not inconceivable that she could trigger when placed in great danger over there (cut off and cornered by S-9, perhaps?).
Cauldron might take interest in a whole new world full of potential soldiers/test subjects. The supernatural powers will suddenly find themselves having to content with strange vials going around and granting regular people mysterious powers.
Imp's power is just an exceptionally strong veil. Is there any conceivable way a Wizard skilled in veils, such as Molly, could pierce it?
Both worlds are enormous playgrounds where the writers simply can stomp around without a care, which makes them very suitable for this kind of crossover.
Based on the stuff I most want to see, the period is likely to be right before Cold Days for the Dresden side (well, probably after he receives the Winter Knight training but before any of the plot occurs, since we want Molly to be stay human for this).
The Worm period would be much earlier, I'd want Taylor to go as Skitter rather than Weaver, she can even have Atlas. And if we want the Travelers involved (and we do), this will have to happen pre-Echidna arc. Since the S-9 would be involved that much, the story could replace/complement their first arc. That means Coil will be the Marcone's main contender for Brockton Bay, for one.