would give a good bit of memory for sound and samples on a 64KB machine
OMG...
Seriously, just use Disks !
According to the Batman Demo, it could perhaps be possible to load while playing... but Game with keyboard/Joystick scan may be a bit trickier than railroaded Demo with no Man-Machine interaction.
Also the PLUS with its audio DMA may be of some help...
But too much Samples are probably not viable because you know, it could need shittons of Extra RAM.
This said, AY sound is quite good enough (if well done) so it could just need to get good versions of various chiptunes and do the movements sequances.
Unanimated screenzone only use a few bits of RAM (as VRAM) and quite no CPU but I see you point in reducing the screen.
But Mode2 is not really a gain... less monitor space for what ? having colours is good to so you don't only rely on shapes to follow the game.
I see you aim at an exact port of the game you posted as video. Well perhaps less ambitious sounds (no fully sampled tunes) and less ambitious animation : not as large thing the side of the whole screen animated...
Many rythm games do exist and I also think this is quite a shame almost none exist on CPC.
Look at
Klax... while not actually a Rythm game, it looks like Guitar Hero sort of stuff.
Ok the game you shown us had a lot of monocolour tiles... (due to the quite large ratio of the comic style graphics) so a correctly designed tile management may not us that much RAM.
To animate this, you "simply" swap tiles.
Tiles could even be compressed I guess.
Also perhaps the Mode0 technic as used for WonderBoy/Ghost and Goblin/ghouls and ghost ... the one with only 4 colours backgrounds and 3+transparency forground...
This method gives large gains in RAM and CPU but can look quite disapointing graphically...
But for this game it could be great actually.
another possible possibly fun possibility thing could be a ability from the CPC to have the Tape read mixed with the sound output.
Like you know, when you load stuff you can hear the bit flow through the Audio output.
Perhaps a way to get this synchronised ?
And to run other stuffs alongside ?