Some suggestions then…
Romans: Almost any range labelled "Early Imperial Roman" is suitable. Perhaps go for a slightly earlier look, with more chainmail than segmented armour and larger or even oval shields for the legionaries. Best avoid 'up-armoured' soldiers, i.e. those wearing bracers (
manicae) and reinforced helmets, since such equipment likely wasn't introduced until the early 2nd century. Other than that, very few limitations.
Germanic tribes/auxiliaries: The Batavians may(!) have looked quite distinctive, even when in Roman service. There are some curious pieces of equipment like metal helmets covered in fur, which are thought to follow Batavian fashion.
This reenactment group provides some useful information. But on the tabletop such detail's easily lost, so you can use any auxiliary to represent Rome's (former) allies. There is even less evidence for Germanic fashion of that period. Again, almost any range of "Ancient Germans" will work. If you're aiming for authenticity, avoid the 'cavemen' look of Copplestone's work for Foundry – although they're quite fun figures.
