Since the trend is smaller caliber and lighter bullet weight is the answer to more power. How low would I have to go in powder charge hence velocity would I have to go to be low enough to come up to the power of the 9MM with a 200 grain hard cast in a 10MM. Since the caliber is fixed and I can't do anything about it, how low in velocity would I have to go to come up with a somewhat close to the 9MM in power. I know I will not be able to equal the 9MM but could I come close?

I have experimented with 10mm. The 10mm with longer OAL can not beat the 40sw with higher pressure possible with small primer pocket.
One mistake is to put a 40sw barrel in a 10mm handgun with triple recoil springs and do an overload work up with 10mm like OAL.... without first throating the chamber. [ask me how I know].

That said the most power I can get out of a 10mm is with 800X , 200 gr bullets, while triple compressing the powder and resizing the loaded ammo.
That is a pain to load. Working up with Power Pistol and 200 gr is easy and almost as good.

It may seem that the max overloaded 9mm with 158 gr and Power Pistol can make more power than the max overloaded 10mm with 200 gr and 800X, but that is because the 9mm is a smaller pistol that is recoil painful.

Not sure if you are describing velocity or energy when inquiring about "power", but the 10mm can launch a 200gr pill at OVER 1,200fps, easily meeting 9mm velocities and well exceeding it's energy.