Allow user to send busses into previous busses for wonderful feedback/audio mulching mayhem… imagine being able to control feedback rhythmically and throwing automated fx into the chain… yum 
It would be amazing, but would send the CPU usage flying through the roof in factors of two. Not gonna happen for a couple of decades yet me thinks.
I don’t see how it would affect the cpu though… Most modular synth programs allow feedback, and if I’m not mistaken didn’t older versions of renoise? 
You are simply taking an audio signal and sending it from one bus to another, sending it from bus 2 to bus 1 wouldn’t require more cpu than sending a signal from bus 2 to bus 3. The feedback is merely an effect on the level of the signal…
Allowing the user to send the signal back to previous busses instead of disabling that option would allow for feedback… 
and much more sophisticated routing possibilities 
I’m totally for this, for feedback to work I think you need to add a little delay between the channels, otherwise you get into a zero length loop and cause the computer to freeze (plus the feedback will really suck). I think that’s what datasette is trying to get at.
The send device could be programmed so that it has a delay in it, one you could change of course. If you make a ‘send’ channel to a previous ‘send channel’ you could make it so that the minimum delay between channels would be increased from 0 samples to 0.1ms or so.
[as a mockup imagine a send device with an extra slider under the ‘amount’ slider] 