A new medium base size could add another depth to learning movements and ranges and things, I am not opposed to it.
But, I'm not sure that 1.5 small base size is different enough to make it make sense it might also add another layer of rules, like for deployment. Heck, it just muddles up deployment since you can't stack 1.5 base sizes in a row, nor can you reasonably fit a small base and a 1.5 medium base in a row because of the numbs on the bases.
Would it be cool? Probably. Is it feasible with the current game and/or worth the manufacturing cost to FFG? Probably not.