As far as starting the IC goes, I was thinking with Rilden's advanced knowledge on beasts, he would create a fake "bounty" on a creature that was very rare, and magically adept, so both Witchers would want to take it on, and Mages (and even High Vampires) would want to tame it...

OR we could use the theme of "fate" and have them all be around the same area as a large teleportation of very hostile and smart creatures that fall upon an important city in the world. Mages would want to see what caused that much magic, Witchers will want to kill them, and so on.

OR we are all connected in some elaborate "we all have our own goals right now" but then they join forces. Like Rilden is chasing a lead on a Witcher/mage sighting, The assassin witcher is taking on a contract, another witcher could be searching for a High Vampire said to be hiding within the town, etc. This one will require a lot of conversation and talking XD

Those are my ideas right now. nothing definite, these are rough drafts, and I'd love to hear your guys' inputs and ideas!