The US M2 Medium was the prototype & basis for the chassis of the M3 & M4 Medium tanks, the M10 TD, the M7 SP howitzer, & assorted other AFV. While there was always room for improvement as a vehicle it was a solid machine in 1939. Its defect when designed and built was the obsolescent upper hull and armament.
What could have been possible for the M2 or M2.5 had the US Army been inspired in the 1930s to build a better armed tank? Would even a lighter 75mm cannon be too much to expect? A M4 like hull with a decent diameter turret ring be beyond the imagination of 1936 or 1939?