As for S6, I'd like to see them make blades that slide into S2 chassis. They specifically said they wouldn't make BM1382 boards for the S2 because it was still profitable, but that's getting pretty tight these days and within a few months probably all of them will be turned off. With a string miner it'll take a bit of finagling to get the power draw right for the internal PSU, but I'd really like to see an efficient DC-DC converter over top of the string with software-controlled voltage output so the user can custom under/overvolt. That opens up a lot of options both for the manufactured stock configuration and user-defined operating points.
+1 I still have a bunch of S2s that I had hopes of upgrading. The upgrade path was the only reason I bought them.