am3+ might support early am4 cpus, just like am2+ did (deneb and thuban both had ddr2 and ddr3 memory controllers)

i doubt we will see every single AMD chip being fusion based in the next generation. that would only work if they built some to have very small gpus and others with more mainstream parts

it could however work very well if they built it around perfect TDP sharing.
imagine a 150W TDP fusion socket:

APU A could be 50W cpu, 100W gpu (low core count, gamer quality gpu)
APU B could be 100W cpu, 50W gpu (medium core count, mainstream gpu)
APU C could be 140W cpu, 10w gpu (high core count, north bridge quality gpu)

then comes turbo and how it might factor in
APU A would have a cpu range of 20-80W and gpu 70-130W (real gpu power when you need it, decent quality cpu when you need it, designed for a budget gamer)
APU B cpu range of 50-130W and gpu range of 20-100W (cpu power nearly as good as the top client chips, and gpu power scalable to almost reach gamer quality, a very balanced chip)
APU C cpu and gpu range very limited. offers the best cpu perf/watt at the loss of any gpu power. designed for tasks that dont need a gpu to do anything special, or where dedicated graphics are guaranteed.