Mh. Depends on your definition of planar.
Byteplanes have been tossed around several times - one pointer for R/G/B each, saving 25% bandwidth. But yes, I agree with your sentiment.
(Ok, one last option: 10 bits planar for HAM10?)
Why, in 2018? Sure, back in the day HAM10 could have been really nice (256 base colours, and true 24-bit (albeit fringed) imagery). But today, memory is cheap, bandwidth is cheap.
It seems to me that an AGA extension today should take advantage of modern memory's burst modes (just like AGA itself increased bandwidth/clock over OCS), and even in an FPGA implementation with limited DDR2/3 memory controller, you could get order of magnitude bandwidth improvements for the graphics system over AGA. Which would mean existing AGA would be unleashed, but also you can implement higher resolutions and higher bit-depths (and if you like: more, larger, more colourful sprites; or more, higher-resolutions, more colourful playfields).