Was adding build timers ever an option or idea?
Personally I originally wanted 0CPs + build time in either T4 or HQ, and it was definitely discussed, but for now we eventually went with the hybrid system. Ideally this would've coincided with changing the cooldown to start upon the unit's destruction, but in the end we found that wasn't possible sadly.
So in the end the call-in production of the heavies was kept, I think for variety and readability's sake (some HQs like OKW's would get a lot of units, it would be odd to have a buildable unit that can be produced only once, heavy tanks are supposed to feel unique and powerful and a call-in does that more justice, etc.), so for now we'll stick with the hybrid system and possibly do some small adjustments (like costs or other characteristics) and let the entire heavies meta settle for now and see where it lands. Making them buildable is always an option for later, if needed.