One of the most interesting bust types used on late roman coins was the one described as: emperor, cuirassed or draped and cuirassed, holds in his raised right hand an ordinary spear pointed up, and in his left hand a shield and two short spears (plumbatae?). Here is an example for Probus: S. Estiot wrote a very interesting article about this bust type between 279 and 307, the example pictured, which sold in 2008, also appears there at p. 197, 8.
LYCAONIA. Iconium (as Claudiconium). Hadrian (117-138 AD). AE. Weight: 2.36 g. Diameter: 17 mm. Obv: ΑΔΡΙΑΝΟС ΚΑΙСΑΡ. Bare head left, heroic from the back. Rev: ΚΛΑΥΔЄΙΚΟΝΙЄωΝ. Perseus standing facing, head right, holding harpa and severed head of Medusa. The heroic from-the-back bust of Hadrian is surprising. Note that two earlier coins of Iconium feature a bust of Perseus depicted from the back. Hadrian has been rendered in the same pose as the city founder. I suspect there was a statue or painting of Perseus in this pose at the local shrine to Perseus. Here is the first coin issued by the city of Iconium: Iconium, 50 BC - 100 AD, AE21
Other very interesting bust variations one can find on the billon/bronze antoniniani of Claudius II Gothicus: 1. a heroic bust from back without spear but with Phobos head on the aegis. and 2. a heroic bust seen from the back with spear pointing forwards and shield, Medusa head on the shield.