1011 posts since 1 Aug, 2006, from Italy
I would suggest Reaktor (it can work as an fx and there's a huge user library), but there's a learning curve (I still have to spend some time on it...). I don't know if sticking to Reaktor Blocks could be a good balance between learning curve and results.
Guitar Righ with Traktor's 12 is probably more hands on and closer to the workflow you describe, but it's also more limited than Reaktor (which can do virtually anything).
I love Eventide's products, I have a few plugins and the H9... If I were you, I'd use something like Reaktor or Guitar Rig for all the crazy fx chains and then look at specific plugins for specific algorithms (you don't need all of them, even if they are cool) to get that "Eventide Sound".
For me it's worth having an algorithm like the H910 (or the H949, which I don't have in my plugin collection); then there are things like Blackhole or UltraTap, which you may or may not dig. As already suggested, you may have a look at SoundToys catalogue (the founders are former Eventide Engineers), for things like MicroShift (which is their take on Eventide micropitchshift) or Crystallizer...
I read more than once on the net that some Eventide boxes were pretty much sitting on the same algo in some studios, so you may not need all the modular power for some effects, but just the right sounding ones, so some specific plugins can make sense.
Given that we are getting close to the sales season, I would suggest you to search for the most known presets/algo of Eventide, listen to demos and see if they fit in your music and then make a list of your priorities.
Just my 2 cents, of course.