The interactions framework sounds like it can do everything the syndromes framework can do, at range and with or without line of sight, plus grant beneficial effects and transform creatures from one species into another, and back again on a timer.
In other words, we should have a rough-and-ready magic system. Area effect and map-wide buff/debuffs, beneficial and baleful polymorphs, you name it.
I think it should be possible to a nightcreature/borg style player race, where you can convert captured enemies to your cause. I can't wait to try this out on the illithids: SPECIAL_EGG to make the illithid's lay tadpole vermin, and a tadpole-injected syndrome to turn captured humans into more illithids.
Tougher megabeasts: It's already possible to create megabeasts that require a whole army to take on, now they can harass your dwarves from outside your fort, should you choose not to go out and confront them. Of course, right now it's not very easy to command your dwarves tactically in battle, so this may not be very fun.
Depending on how vampires get implemented, we may finally be able to make creatures at workshops: Toady said the vampires should have the ability to transform into vermin and back again, and we can already make vermin at workshops! So we may be able to make a vermin that will turn into golem or whatever immediately after spawning.