I doubt Data will be back. I mean, it's already been made quite clear that B-4 is technologically no way near as advanced as Data was. B-4's positronic network is just not capable of handing the sophisticated programs of Data. They'd need to build a better positronic brain, something that Federation technology is just not capable of yet.
Didn't they replace a portion of a humanoid brain with a positronic one on DS9? If so, I'd say the technology has grown quite a bit over the years.
And just because the hardware isn't as advanced doesn't necessarily mean its incapable of running the software. It may simply not be as efficient.
I think it was stated in Nemesis that B4 would never fully integrate Data's datatransfer, his technology simply was to...simple, compared to Data.
As for the half humanoid/half positronic brain, that was Vedek Bareil. He died a few days later....