Interesting that you say the Churchill offers nothing a T-34 or Firefly can't do, then offer up a T-34 look-alike (in game terms, anyway) of the Cromwell and a Panzer look-alike (in game terms) in the M-10?
The Churchill would offer more in terms of game variety than either of these two. It would be a slow, heavily armored, medium armed tank.
We have a fast, poorly armored, lightly armed vehicle (M-8)
We have a medium speed, poorly armored, lightly armed vehicle (LVT-4)
We have a medium-high speed, medium armored, medium armed vehicle (T-34)
We have a medium speed, medium armored, medium-high armed vehicle (PzkwIV(H))
We have a medium speed, medium armored, heavily armed vehicle (Firefly)
We have a medium speed, heavily armored, heavily armed vehicle (Tiger).
So, the Churchill is something we do not already have.
Your M-18 has merit because it would be fast, lightly armored but medium-high armed (something we do not have).
German/Russian TDs of any kind would have merit because we presently have no "turretless tanks."
I don't see a great need for an M-10 or a Cromwell. Similarly, I see no reason for a tank like a Pershing, which in game terms would be just like the Tiger. Any of these might be "cool to have", but given the very limited scope of the ground war, I would love to see more actual variety of capability rather than purely catering to a "cool factor."
My two cents, opinions may differ.