That looks good, Markus. I don't think you need the extra 68K. You could just use the pad that the jumper goes to. In fact, maybe that would be the spot to put the switch. I think this might also be the spot to put in a wet/dry (vibrato) control option, replacing the two 68K's with a 150K pot, if desired. If you add the stereo mod, the two 68K's could perhaps optionally be replaced with a dual pot to control the balance between the two outputs, if you want.
Another option that would go well with the vibrato option would be to add a dry, direct output. Again, levels might have to be tweaked, but a switching jack that would break the connection on one side of the 68K thats attached to IC1b would do well (at Markus' TZF jumper). This would also be like an easy psudo-stereo type mod.
Here's another interesting read on making stereo...
http://www.diystompboxes.com/smfforum/index.php?topic=11254.0And, I'm gonna dredge this up 'cause it's a really good discussion...
http://www.diystompboxes.com/smfforum/index.php?topic=25681.0... and, there's some good info in there on opamp selection, among other things.

I've also come across a few posts in the archives that talk about a level change when the effect is active, versus bypassed. I'm going to guess that if the effect is modified for true bypass, this could be an issue? So, might it make sense to maybe replace the output buffer's 4K7 feedback resistor with a 10K trim pot, or some other means of adjustment for output level matching?
I've also found some talk about the configuration of those diodes in the input section... I'm sure y'all already knew their purpose, but I was very interested to learn that they act as a limiter that tames the regeneration (enhance) at extreme settings, or something like that. Anyway, it seems that the older ~Rev3 configuration is preferred.
Also, another interesting comment about the threshold control...
The "Threshold" control is designed to cut off the delay mix section wen you're not playing. Many flangers continue to "whoosh" when you are not playing, but the A/DA is dead quiet. If you run the threshold control all the way up, it takes a pretty hefty signal to kick the flange back in but I wouldn't say it's something you can play with like you could, say, an auto-wah. Not really in the same league as far as interactivity goes.
But, there's still lots of reports out there about how quiet this circuit is without the threshold control. It would certainly simplify the build to leave it out. And, I think it takes up too much space to leave as an option. So, I'm still mulling it over. Anyone have any other thoughts about this?
Yes, I have a variable capacitor in my ADA Flanger.
Stephen
Sorry, Stephen. I know that's a pretty old quote. But, it just made me curious... do you have any recollection?
Thanks!
~ Charlie