The Hetzer upgrade got me looking into what were proposed for the 38(t) chassis, and I came across this gem of an idea:
The 8cm gun PAW 600 is actually light enough to be fitted onto the 38 (t) chassis without overloading the suspension (some 200 of these smoothbore guns were made before the end of the war), and it would've fired HEAT rounds. This would've been effective in engagements at around 700m or so. Marrying this gun with a very reliable chassis was immediately on the drafting table, but the war ended before it can come into fruition. That gun has enough penetration to be equal to that the Panzer IV with the L48 75mm, firing Tungsten rounds.