Sure, but there are cases where is more convenient to have the nd's in the body, specially for lenses with bulbous front elements, probably like the upcoming Canon RF tilt shift lenses. Also internal nd filter systems don't have any reflection issues like front filters.
Ibis is good but If I'd need to choose, it would be ND's over Ibis, any day. For people that want Ibis, the regular R5II is going to be the one
Sure, it would be infinitely more convenient. ND filter makers certainly would not approve this change I'm sure.
But then it would not be an R5 C Mark II anymore at all.
You would need a completely different body to have spacing for the turret mechanism that rotates them, and you probably need to remove the mechanical shutter as well, heavily compromising stills capabilities. (but cameras like the Z8/Z9 might become more popular, so this may not be as relevant in the future)
A small thing, but I guess this would probably require more power as well which is already right at its very limits with the current battery regarding its "party piece" 8k60p RAW recording option...
So basically, you'll end up with a
cinema camera rather tihan a
compact stills camera with advanced video capabilities - mainly via the software, otherwise it is just active cooling instead of IBIS. And all this for a
moderate price increase. People wouldn't consider it otherwise, especially if they make a C90 or something (I'm assuming they need more space for the ND filters over C70 but at least the 17:9 sensor aspect ratio is better than having the full stills area)
An electronic variable ND like Sony uses would be neat, only requiring two positions therefore less space (especially useful for a full-frame sensor) and some useful tricks, but Canon does not have this technology. Also, it would be around 2-7 stops, so not quite the range of Canon's 2-4-6-8-10 stops ND. So it may still need additional ND in some cases, which would be very annoying.
Point is: internal ND filters in "stills cameras" is probably not that easy - but also get easier as time goes and manufactures are in no rush to implement it (to segment their products much better regarding features and pricing).