I'm on the lookout - there are scores upon scores of fantabulous free and paid synths out there, so I'm sure someone knows what I want:
 
I'm looking for something in the vein of a combination of Dropzone (software that lets you load in your own sample, set loop points and stuff to make the sample loop properly), combined with a live audio-processing synth.
 
Essentially I want to send an audio signal thru a synth that can "reskin" the notes I'm playing with a sample in realtime - particularly a sample, not a synthesized sound. Best if it can take custom samples.
 
Is it out there? :)

If I understood your question correctly, I'm not aware of anything that can play a different sample based on incoming audio (notes) in real-time.  MIDI yes, audio?
 
The only things I'm aware of so far in this regard is for using a synth as an effect, or as a vocoder, to re-synthesize incoming audio.  But as far as triggering new samples, that certainly sounds interesting, but don't know if that has been achieved yet.  I would see the initial challenge as doing the pitch detection, aka ARA Celemony Melodyne, and then substituting a sound.
 
If anybody can correct me, I am interested in this topic as well!

abacab
If I understood your question correctly, I'm not aware of anything that can play a different sample based on incoming audio (notes) in real-time.  MIDI yes, audio?
 
The only things I'm aware of so far in this regard is for using a synth as an effect, or as a vocoder, to re-synthesize incoming audio.  But as far as triggering new samples, that certainly sounds interesting, but don't know if that has been achieved yet.  I would see the initial challenge as doing the pitch detection, aka ARA Celemony Melodyne, and then substituting a sound.
 
If anybody can correct me, I am interested in this topic as well!

The closest I've come to seeing it is with things like GCBS4. But I guess it needs to be quick/low-latency enough to take a note played live, detect the pitch and then convert that to MIDI all within a  reasonable enough amount of time that it feels more or less real-time (depending on the intended use).
After all, one could always just play the part on a MIDI keyboard or drop audio onto a MIDI track and let Melodyne or other software figure out the rest. But then that wouldn't be any fun, now would it??
 
My idea in the long-run being of course to be able to record some trumpet or 'bone, and then have the audio shuffled through a synth/sampling software that can vaguely emulate the sound of a saxophone with a little more natural expression that what you can get with standard MIDI playing on a keyboard by a fumbly semi-keyboardist like myself 

I'd guess there are two aspects here, the transient and the tone. As you say "vaguely emulate the sound of a saxophone" then I maybe yu culd shape the transient real time with a transient shaper designed fr the task, in rea time. For the tone, you'd be looking to change the shape of the wave form and that would require mdifying the harmnic series in real time.

 
To do this would require real time pitch tracking, then Timbral shaping of the harmonic series. Autotune does real time tracking,  and melodyne timbral shaping, so I guess the technology is possible. How effective it would be I don't know. I haven't tried the timbral shaping on brass to make it sound like a reed in melodyne, but I have sample modelling's brass collection, it it allows shaping of the first 10 partials. But I can't get the brass sound any different than brass with it.

https://pages.mtu.edu/~suits/sax_sounds/index.html
 

It sounds like all you need is a realtime audio to MIDI converter. Once you have that conversion happening in real time then it's just a matter of routing the MIDI to whatever synth or sampler you like. I'm not sure but I think that Melda's free MTuner converts audio to MIDI notes in real time. I would think you'd just enable MIDI-out in the VST settings, then set MTuner as the input to the MIDI track that's playing the synth. Having said that, I don't know if it outputs velocity as well as pitch. That may or may not be an acceptable limitation for you. Melodyne outputs velocity, but of course that's not in real time. 

Meldyne used to be able to output pitch too, but they removed that function with 4. I guess there were some issues, but I hope it returns in the future, and they add expression, maybe from RMS.

Maybe a MIDI guitar plugin?

sharke
It sounds like all you need is a realtime audio to MIDI converter. Once you have that conversion happening in real time then it's just a matter of routing the MIDI to whatever synth or sampler you like. I'm not sure but I think that Melda's free MTuner converts audio to MIDI notes in real time. I would think you'd just enable MIDI-out in the VST settings, then set MTuner as the input to the MIDI track that's playing the synth. Having said that, I don't know if it outputs velocity as well as pitch. That may or may not be an acceptable limitation for you. Melodyne outputs velocity, but of course that's not in real time. 

Cub-ase has all this already (the audio to midi to note)

I am not sure if I am understanding the OP correctly. Seems you want a sample editor that will then play samples to the pitch of each MIDI key and allow you access to underlying synth parameters? The first thing that comes to mind is iZotope's IRIS 2, which is worth checking out (free 10-day trial). The sample manipulation is pretty good with that, and the rest of the audio engine is synth-based (you can also layer up to 4 samples and determine loop points and how it is played (start to finish, back and forth, finish to start, etc.)). That goes on sale fairly regularly and has upgrade options too.