Apple's SoC development wisely tracks IMG's roadmap for PowerVR cores, so the A6 will be the incremental upgrade, using the same core but upping the implementation: clock rate, bandwidth, etc. Conceivably, using the same 543MP core, they could up the implementation by even slotting in extra cores this time -- a 543MP4, for example.
Adding more cores could only be done practically under a sub-45nm process, of course.
The graphics jump from the original iPhone's A1 SoC to the iPod touch 2nd generation's A2 was basically a GPU clock speed bump from like 103 MHz to 133 MHz, but the jump to the 3GS's A3 SoC and its ISA change of ARMv6 to ARMv7 and MBX to SGX changeover was just massive.