The griffin was associated with the Thracian, and was depicted on Thracian helmet-crests, so it might well have featured on his buckler too.
The Myrmillo was traditionally the champion of Neptune, so fish, dolphins, hippocampi, and horses. (Neptune being the Lord of Horses as well as matters maritime.)
The Hoplomachus was originally Greek-inspired, so Greek-style imagery wouldn't look out-of-place… in the same vein, early Italian symbols seem appropriate for a Samnite.
Generic symbols of contest and victory, such as laurel wreaths and 'Winged Victories'.
Some Gladiators adopted stage personas derived from mythology, so a fighter identifying himself as 'Hercules' could display a Theban-style club, for example.
