Celts, that means britons. Lorica segmentata appeared after Augustus, in time for the british conquest and Boudica's revolt. Then - picts.

Germans. Germanicus, the father of Caligula, fought them and made his name with his victories.

Parthians. Trajan had a little campaign against them, after the dacian conquest.

Hebrews, illyrian, with famous uprisings of large armies.

Then - of course, other romans, as civil wars were very common between different pretenders to the throne.

Good question Misteredd, german foot to go with the cavalry would be a start.