You never need a new unit just because there needs to be a new unit; you only need a new unit when there is some sort of gap in the units currently in existence. In my personal opinion, the only gaps in the current unit set are:
- the difference between American-style tank destroyers (fast and lightly armored with a 360 degrees turret, like the Hellcat and Wolverine) and the German/Soviet style assault guns (slow and heavy armored, without turret, like the StuG and SU-types),
- the difference between long-range naval scout- and sub hunter planes (Catalina, Condor, etc) and the short-range surface attack dive- and torpedo bombers (Devastator, Kate, etc).
From a game play perspective, I'm not sure if these splits should actually be made. Less is more, you know.