A Delay where delay is created by varying the samplerate, as in BBD analog delay devices. A FIFO stack makes sense in my mind, can also use a ton of Sample and Holds to do it though. Only 3ms to 15ms would be enough, so that we could build true analog style choruses and flangers. PS - the DAZ Bucket does come very close, sleep mode and interpolation would be nice though.
PSS. A LIFO stack would be cool for going in reverse.