Are there any major Sherman variants that have not been done in kit form?
It all depends what scale you are asking about. None of the funnies have been made in 1/35. Nor a M4 105mm. In 1/48, most of the series has NOT been molded. Not sure in 1/72.
Early M4A1 with sandshields for one - in any scale AFAIK
Fujimi did the M4 105mm with HVSS suspension in 1/76 scale.
Fortunately for Sherman fans, there have been many different aftermarket sets to create many different versions of the tank.
In 1/48 scale, Steve Faxon has produced many conversions for the old Bandai Shermans and has since switched to the Tamiya and Hobby Boss kits.