Paul –
It would seem then that you rule out castings. If parts of whatever you publish can be 3D printed or CNC cut (laser or water-jet) you may well have more choice of suppliers; and at least one advertises in the magazine.
Designing in metric, fine. If a builder has only imperial machines, micrometers, etc, he or she can convert the numbers as they go. I use either system according to project, converting as and when necessary.
However, the thread has rather wandered off from your question: can we have new designs of models?
I for one am not certain what machines you wish to offer, for a start.
You can design as many steam-engines, i.c.-engines, hot-air engines, water turbines, gas turbines… as you like; but really, all these are re-inventing if not the wheel then certainly very familiar objects.