Well, Warlord’s Orcs could work as rough auxiliary troops or opponents for Fantasy Romans since thier armor looks close enough to looted legionnaire armor a paint job and shield swap might cover it, similar deal with Wargames Atlantic’s goblins, plus you can always take WA goblin heads and stick them on Warlord Hoplites, legionnaires, and Celts for orcs (not to mention WA’s own late roman sets).
There is also RDG Fauns, Satyres, and Centaurs plastic sets if you can find them which would work great for Republic and Early Empire auxiliary forces or enemies as would Wargames Atlantic’s skeleton hoplites (cause its Mediterranean) and even their spiders (which work for everything).
There are tons of metal options as well, which to be honest you can get different variants of depending on maker. My recommendation is remember that you can use a lot of Greek, early Celtic, and even Egyptian and Persian stuff as well “D&D race X in Roman gear” depending on what part and time frame of the empire you want them from, and with that in mind decide what you want to shop around for first weather that be an exotic non-human heroes like an orc general or big humanoid monster like a Minotaur.
Beyond that you can always paint human minis into ghosts/undead like my WA goth in this photo:
