How fierce Rome truly is, the Celts do not have an accurate perception, but if Rome indeed grinds the Germans into the ground, then the next target would definitely be the Celts.
After all, the hatred between the Celts and Rome is not to be taken lightly. This Barbarian claims to be a mercenary, but Rome has had little opportunity to hire the Celts; they are always hired by Rome's enemies. Historically, the ancestors of these Barbarians even crossed over to participate in the Punic Wars.
There were even instances when they seized the opportunity to loot Rome while Rome was engaged in conflict with other nations; the hatred is significant, just slightly less than a blood feud.
As for the Slavs, a warrior nation needs no explanation. Their hatred with Rome isn't that great, but if Rome does defeat the Germans and Celts, there's no doubt the Slavs will have to submit as well.
