As far as the perforated drum or the solid drum, to me it's about influencing the heat transfer. For a given metal and thickness of drum a perforated drum should respond to temperature changes quicker. The second part is the ability to change how much of the heat transfer is conduction vs. convection. Fluid/spurting bed roasters swing the pendulum to convection, while solid drums with little direct contact with moving gas/air swing it to conduction. In my perfect roaster, I would be able to decide. Radiant heat transfer is another possible influence to play with, but I think it would be harder to vary.
I think that baffling gas flow could have a similar effect, but care needs to be taken to keep the heating even. I wouldn't want hot and cool spots, especially if they were inconsistent and changed with air flow adjustment. The other part of this is that having a thermally responsive drum and a thermally responsive heat source could make for a finicky roaster.
This doesn't have to be a drum roaster, right? An SCTO'ish style roaster could incorporate many of the wish list items.
We should also consider energy efficiency, safety devices, chaff removal and style.
Energy Efficiency:
- gas/air re-circulation (or regenerative air heating) could increase the incoming air temperature and reduce fuel required.
- Proper insulation of the roaster to keep the heat in.
Safety:
- Interlock the fan to shut down on fire detection
- Thermocouple looking for rapid heat increase that could initiate an emergency shutdown
- The ability to dump the beans to a cooler without getting burnt
- Minimizer hot surfaces to prevent burns and possible external fires
- There are probably other codes that would have to be complied too (especially if gas was used) and a UL rating would probably be necessary
Chaff Removal
I think chaff removal is a design consideration for later after some other choices are made, but it is important. Poor chaff removal can create a fire hazard and, at a minimum, ruin the appearance of beautiful beans. This is not a big deal for me, but a new roasting hobbyist or customer who gets a bag full of chaff might disagree.
Style
Not really my area of expertise, but style can make or break a product in the mass market. I've seen useless devices sell like hot cakes because they were stylish and vice-a-versa.