Well, yes. That would be terrific.
- No more attribute clash, nor a second memory area to cope with.
- 1 byte per pixel. More memory to move around, but the DMA could help, and pixels locations easier to calculate (3 or 5 banks of 16k RAM laid out horizontally (256x192) or vertically (320x256), all in order without interlacing. 256 colours palette from 512 colours RRRGGGBBB. No memory contention. Textures possible, as well as more lighting gradients than just "dark or bright"... :p
- Counter not necessary, the Copper can be used to synchronize with the raster. Z80 CTC timers onboard, providing interrupt vectors for true IM2 management if needed.
- Multi-layers possible with transpacency, logical "and" mode, etc.
- Hardware sprites available. Would maybe change the principles a lot from an original Spectrum, though...
- Possibility to run at 3.5, 7, 14 or 28 MHz.
It's true the programming techniques would probably differ a lot because using more colourful objects would mean having to rely more on the new hardware to compensate. But it would result into more readable and aesthetic effects...
However, the optimized maths would still be very useful. It would allow more complexity without sacrificing fps. :)