There could be upgrades to the camp to make the troops veteran. Personally, I like that the troops trained are higher level. For the sake of creativity, the camp could slowly heal troops inside at the cost of an upgrade. To circumvent the "abuse" of mobile arsenals, the production of rams could be slower in the camps. Also, nobody uses the siege wall. currently, garrisoning of walls is one tactic I see no one use.
Are there any thoughts to this?
Also, on a side note, didn't the romans have slingers too? I think it would upset some balancing if they were a stock troop but if there was an upgrade in the barracks to allow the production of slingers, the romans would have more versatility. One of the worst things about playing Romans is playing in low wood environments. They don't have mercenaries or slingers so it makes Romans extremely disadvantaged in these biomes.