We do not have a Sherman because without a numerical advantage, it is very limited. And it isn't "glamorous".
We already have what amounts to an upgunned Sherman (and I suppose slightly uparmored), and a plain short barrel 75MM gun Sherman, while easy enough to add, likely wouldn't be real popular. Not to say it shouldn't be done.
The M-18 is something of a late war beast, and would probably be real close to being a perk ride. You'd have to know how to use it well before it'd be worth it if perked.
A Panther isn't a bad idea, neither is a Comet. But really, isn't a Panther, being (somewhere around) half way between a Panzer and a Tiger, about where a Firefly is? Not that there is anything wrong with that. At least with regards to guns, the Comet is about where the Panther and Firefly are.
If you had the Panther and the Comet, that would round out the set of the Firefly level, well, I guess if you added an upgunned T-38 as well.
The M-18 would be in a class all its own.
What you'd lack then is an Allied (but non Soviet) counter to the Tiger, if you wanted that, you'd need the Pershing.