Opinions expressed below are my own, feel free to comment / agree / disagree.
We should be careful that the extension isn’t too prescriptive as this is really the specialist field of the Mechanical Engineer and requirements / conditions will differ between sites / jobs / areas. This extension could come in two flavours: Engineering (Pro) version (that might be able to do the required math), and Planning (Lite) version where you can route / block out rule of thumb requirements input by the operator.
The range of mechanical equipment is probably too vast to cover and may bloat the extension with all the libraries required.
Having said that, a system that you can size/choose HVAC ducting (rigid square, rigid rectangular, rigid round, flexible round) from commonly used sizes would be beneficial. The input method should also be flexible (offer opportunity for manual input). Don’t forget the space surrounding ducting for required bracketing to both keep the ducting together and fit it to the structure (they are different components). Also, offer option of customised thickness of insulation (sound attenuation and thermal options).
When it comes to grilles / diffusers, the same type of flexibility in interface design and options should be considered. A range of generic components (possibly dynamic components?) would suffice.
If one wants to exchange these for product specific ones then the user could do it as they see fit with models / components they supply. This may also be a possible revenue stream to get manufacturers to supply these models (or link to online repository in the background) and “advertise” using the extension. However the extension should not get locked into any specific brand / supplier.
Obviously, as per Medeek extensions usually spoiling us, be able to assign custom materials & layers (now tags since SU 2020) and custom layer / tag names to the various levels / kinds of entries.
*** EDIT ***
These don’t need to do auto cutting of walls / ceilings / whatever, that function can be performed by the operator using the tool of their choice.