a new internal instrument but not to generate sound itself but to contain, combine some other instruments, internal sampler or vsts… so you need to fix audio from each “children instrument” contained in this layer before composing…
about the internal sampler, that means adding a “fix / free” audio out for each sample… but it’s possible i think… or just only to use vsts…
I’ve found myself seriously wanting one of those too.
yep… kind of complicated for sure…
Still the same problem for me i think… 
This way is the “write down before hear” if i understand well… your idea means you can “pick up” the sound from any track then route to others containing listeners…
well, seems cool but what i feel really simpler is , as i allready sayed, having more than 1 synths/samplers on the same “keyboard//LAYER/INSTRUMENT”.
I really think it’s more more simple and intuitive, cause you can directly hear what you’re gonna have + you can, for example, attach new children later while your pattern is playing and edit them so they can responds with the velocity/pan/note/etc.(or other parameters. Ex: with hydra, track crossing automations, morphing by crossfades, etc.) with different responds/ enveloppes, etc.
About the pattern commands, all i can say is that my suggestion allready exists in some other trackers stuffs that i also use, but still in beta phase.(like psycle for example) so it’s doable here… 
(ex: psycle 1.8. changes: “Added a new internal machine “Note Duplicator”. It just resends the Tick command to the specified machines”.)
This note duplicator can only choose between octaves and is only able to trig all attached instruments at the same time, (so it’s not really powerfull ), but each instrument (+internal sampler!) can be routed anywhere, reverbs, flangers etc and edited separately
and:
THE PATTERN COMMANDS WORKS DIRECTLY WITH THE “TRIGGER”, VELOCITY, PAN, 01XX, 02XX, ETC. … SAME AS IF YOU WERE USING A SINGLE SAMPLER/INSTRUMENT!
You also can feel free to use a single note duplicator on few traks to be able to use more pattern commands… (ex. 01XX, sample forward backward… etc)
So my vision means:
the same but with few features added, like keyrange for the trig of each “child”, the pitch, and maybe that’s all, because every other parameter, like pan, velocity, retrig, sample offset (for the sampler) works directly with the “layer”
in this case, the little option of adding a “fix / free” audio out for each sample can easilly get understood because it’s gonna complete this new feature…to become really powerfull…
i think this suggestion is simpler, and really powerfull regardless to it’s simplicity… really much faster to do (compared with all i can read here in ideas and suggestions, but i surely missed something…) and quickly into our home studios 
I think we need Max/Msp, PD, Reaktor, Buzz view for all kinds of routing, linkage of control (be it control &/or sound data) just to make it visually more clear what is happening and ease of connecting stuff. To set-up send,‘listener’ devices etc in the Track Dsp’s tab is cool, but pales in comparison to having the flexibility & visualized something like this:
I suppose you know this but still, for the moment the only layering that one can do is by having both sample(s) and a VSTi assigned to the same instrument slot. Thus the VSTi and the samples play at the same time. Not very flexible but I’ve made good use of it on occasions. This is something you could experiment with while waiting for proper layering.
yep! i knew it … that’s what i often do, a good way to wait “THE” issue… but kind of frustrating
YEP! MODULAR VIEW IS DEFINITIVELY NEEDED
I think something like this is urgently needed.