I like how you think. One would guess it might be possible to have a 24mm D5 BP motor - something with that nice initial hit of the C5 but a really long tail. I would guess that would not be much heavier than a longer 18mm motor.
I am not a motor expert and do not play one on TV.
That might be doable. AeroTech manufactures a D2.3T in an 18x70mm case that is 4g heavier than the C5, has slightly less peak thrust than the C6, and has no ejection charge (requiring electronic ejection in standard rockets). There is also a C3.4T in the same case with a shorter sustaining thrust.
Granted, these motors are meant for rocket gliders, but expert opinion in one of my other threads indicates that electronic ejection in an BT-20- or BT-50-based standard rocket flying on such motors might be doable.
I’m guessing that an extra 25mm of paper casing and propellant/delay/ejection grain would be comparable to or less than the extra weight of the electronics and metal reloadable hardware, and that the C5-type partial core would improve liftoff performance enough to allow common use. Offering 4, 6, or 8 seconds the way the E12 motors do should present plenty of delay options.
So the motor itself may be viable, but they’d also need all kinds of new tooling to make them. They would also need to create or modify new kits with a longer mount to take the motor, all of which would have to be tested extensively. Then there is certification to think about.
Can the market for the motor support all of this? I’d call it questionable.
A far smarter option would be to reintroduce the B8 from some decades ago, I believe that had the same nozzle and coring.
They work ok but there are a limited number of rockets that can use them due to the short delay. I feel like a C5-5 would be more useful as it would open the motor up to more rockets.
In all honesty, when something gets border line for the C6 motors I generally go up to 24mm on the mount so I have the option of the C11 and D12 family.
I don’t see much utility for a C5-5. The C5-3 was essentially designed for rockets that are tricky fliers on a C6-3. It’s basically their way of getting heavy rockets up without having to shorten the delay any more than it is.
Anything that flies OK on a C5-5 would likely also do well on a C6-3.