These examples were all plucked off acsearch.info and are thus from auction catalogues. They just supplement the many images available on CRRO for RRC 319/1. Anyway I just want to note that the right hand figure, which Crawford identifies as a barbarian soldier is probably meant to be a Macedonian. The round shield is one hint. As are the horns on the helmet, this may even mean a royal Macedonian is meant (see earlier posts). I think the strong diagonal element across the figure’s chest may be trying to represent a chlamys (Macedonian cloak).
Cf. Alexander’s chlamys on the so called Alexander Sarcophagus
And the left hand figure’s chlamys in this Pella pebble mosaic:
Liv Mariah Yarrow 
I have a slightly different guess, which you could say is an intermediate stand between Crawford’s and yours. I think that just like T. Didius (RRC 293, which does not represent gladiators at all!) a few years earlier, M. Minucius Thermus might primarily wish to allude to the recent victories of a member from his gens (M. Minucius Rufus) over the Scordisci and the Thracians in 107 BC.
Not all horned helmets allude to the Macedonians or their kings: these have to be specifically caprine and are represented on Roman coins as “twisted”, which is not the case here. Conversely, bovine horns are associated to Thracians by Herodotus 7.76; by the time of Caesar, for some reason, horned helmets seem to allude more systematically to barbarians, as implied by the trophies featured on some of his denarii. Besides, I think that pretty much all monetary depictions of single battles on RR denarii invariably feature barbarians.
My guess, then, is that the Roman engravers had quite a hard time in choosing how to represent “Celticized” Thracians -or wait, is it the opposite?- as there was no common representation for these tribes neither in the hellenistic nor in the Roman repertoires. They found it suitable to represent a classical rescuing/battling scene against a barbarian, as inspired by hellenistic gigantomachies, while mixing “exotic” features (the horned helmet) and (Northern) Greek ones (the chlamys and parma). After all, by the 1st century BC, this is exactly what happened with the “Thracian” gladiators, who had little to do with actual Thracians: they were mostly depicted as fearsome, treacherous barbarians with some pieces of Greek weaponry (the parma, again).
Anyway, you can look it up on Academia if you wish: http://tinyurl.com/h6xv939 (p.52-54).
Thank you for this very detailed comment! I like your ideas very much and must think more about them.