In 1991 Commodore realized that AAA cost was going high, so they postponed it until 1994 and hesitantly designed AGA and released it in 1992 to keep up with the competitors.
According to Dave Haynie AA+ only existed on papers and the actual design never started due to Commodore's lack of money at the time.
AA+ systems would be shipped with the forthcoming AmigaOS 4 which added RTG support for chunky pixels.
For Chunky pixels support, low end systems would most likely feature a 68020 with full 32-bit memory addressing (i.e. not 68EC020) or even 68EC030 which could handle RTG drivers easily.
With 57 MHz pixel clock, AA+ could display progressive 800 x 600 @ 72 Hz in 256 colors, or even interlaced 1024 x 768 screens.
A 2X performance might be gained by increasing blitter clock cycle from 7 MHz to 14 MHz, but by doing this AA+ will lose compatibility with a large base of hardware banging software which depend on synchronizing with blitter cycles like most demos and games of that era.
When asked, Lew Eggebrecht VP of Engineering at Commodore stated that AA+ will support 16-bit sound samples, but it is unclear whether this support would be added by adding a DSP chip, or by improving Paula to something better like AAA, although Lew Eggebrecht stated once that DSP will be integrated in all future Amiga chipsets including the low end ones.