M4 SHERMAN Would most likely be the best all around model to work on for the next vehicle to be added. Not just for the numbers that were used in WW2 or that it was used on both ETO fronts, MTO and in the PTO but..... if AH took the time they could get 4 vehicles on one hull.
You could have the standard M4A3 Hull with different turrets to get more vehicles:
M4A3 Sherman - 75 mm gun (Free)

M4A3 Sherman - 105 mm Low velocity Gun (Free)

Sherman Firefly - 17 pdr gun (Slightly Perked)

M36B1 Jackson - 90mm gun (Higher Perked)

BTY, The M4 Sherman's Road Speed was only 30mph and the M18 Hellcat was only like 55mph. Both were slower cross country.