Unbuilt Stirling models
While the other British heavy bombers had multiple alternate models designed, the Stirling as the "backup" model had very little further development undertaken.
Short S.34 "Ideal Bomber" Stirling
In the late 30's British dogma assumed that a heavy cannon defensive armament would be required to defend large aircraft from enemy fighters, and work was undertaken designing the massive four-cannon dorsal and ventral turrets.
In response to the "Ideal Bomber" Specification B.1/39 Shorts tendered 2 modified Stirlings, a single tailed version with Hercules engines, and a twin-tailed version with Griffon engines. The wings would be mid-mounted and have extended span. Unfortunately no illustrations of these designs appear to have survived.
Short S.36 Super Stirling
By 1941 it was obvious that the Lancaster and Halifax were superior to the Stirling, and so Shorts started work on significant improvements to the Stirling, initially refered to by Shorts as the Stirling Mk.III.
The basis of the modification would be the use of Bristol Centaurus CE.3.SM engines, with redesigned wings with wing bomb cells, and an extended fuselage featuring an enlarged bomb-bay. The performance of the new aircraft was better than the Stirling Mk.II, while carrying almost twice the bombload. Two prototypes were ordered in January 1942.
However the change to construction of the new type would mean the production rate of Stirlings would drop by a half, and the Air Ministry decided that resources should instead be devoted to updating the basic Stirling to use the Hercules VI engine, leading to the Stirling Mk.III that we know.