There is Major reason why i fear vst might not make it to individual layers or riffs and that is cross platform/cross user compatibility.
Having vst or au at I/O is simple, the host(os specific) keeps track of plugin being used, and your songs/riffs are thus independent of the implementation and hence cross platform.
As soon as a vst's/au get into riff/layer level, it is not merely a can but rather a cauldron of worms and what not.
Rifflink collabs won't be possible due to different plugin standards and plugins amongst users. There will be major compatibility issues etc. and will cause problems amongst most of the users.
A Little bit of freedom/routing in the internal fx section would be great. Like the order of fx used. and few more fx perhaps including a limiter on the layer level so a offending layer does not make the entire riff huffing and puffing .
a vst/au section at output would be a good option, even the inbuilt 7 fx stack set at final out would be a great option for tweaking up the overall mix/master.
One can go a lot deep into it and have submix tracks and fx sends and all but that would make RW too complex, not a good thing imo
Since midi support is coming soon to RW, i hope Midi to Rewire Routing is also implemented (if not now then atleast in future versions) , as it just does not make sense of incomplete usage of rewire protocol for just using loops/Pre recorded stuff in RW (as RW can only act as a master) .
With Midi Routing,You can record your midi clips in your rewire slave right from RW. and even use vsti's or vsti/vst chains that maybe incompatible within RW itself.
My most desired feature, is in the rewire/rex/ID usage : Bounce/mix to layer instead of to a new riff. It's a pain to setup rewire everytime you load a project, and mixing down the entire riff is quite destructive process. Like you have to follow a rule, that if you want to get the loop from rewire or rex intrinsically into rw, you have to mix it down first, before laying down any other layers.
Why would you want to mix to layer? because then with info view you can control the mix of the loop, mute individual hits etc, merge 2 instant dummers like taking loop from one intensity setting and fill from other setting.Or perhaps get rid of the entire loop if not required in final mix.
A different vst/vsti implematation could be great like a vst chain/stack on input built right into RW vst loader. Again one can use freeware/commercial vst loaders and most work just fine so not a biggie.
Most of above ideas would/should not turn RW into a High End DAW/Recording App Or make it any complex, but most likely make it a bit easier for all as these are just bits of extensions to what already is in there.
Hope with time some of these small things would make their way into RW.
Nuff for my Sunday typing practice (read:finger therapy) Session

~Amit
I Am A Man of Few Words Ironically Understandable by the Select Few, hence more often than not I have to Come out of my Manhood to do the obvious. Now Ain't it a cruel world?