Legion reinforcements

[starman2012]

I know I'm a little late on this but I strongly agree with your statement about them coming from the same region to avoid ethnic tensions.

The potential downside is that this might encourage separatism when the whole point of conquest was political unification.

Romans felt that the Gauls (and many others) were inferior and I would not doubt that there would be conflict within the legion putting it on the brink of mutiny.

Down to the third century, it is thought that barbarians recruited into the army, such as those captured by Timesitheus in the balkans, were used to fill gaps in existing Roman units instead of forming their own.