Problem: Roman Garrison still is not very useful, largely because it does not offer any upgrades or units which cannot be made elsewhere. Furthermore, legionairres are severely outclassed by fully upgraded man-at-arms, which makes them a very illogical late game infantry alternative.
Idea: Legion Training Tech. This tech would cost 5 turns to complete, and can only be researched in a Roman garrison. When researched, it gives all legionairres this ability: All legionairres get +1 in armor and pierce armor for each adjacent legionarrie, up to +4 defense if a legionairre is surrounded by 4 other legionairres. This is historically accurate, as the Roman shield wall was nearly invincible for its time.
Result: The Roman garrison becomes more useful. Roman legionairres become a viable strategy even late game, but require careful strategy to be used effectively.
It would not be overpowered, as you would need to
1. Build many legionairres to use this effectively
2. They would be grouped together, which makes certain weapons like ballistae a strategic counter to them.
3. Even if you had 7-8 legionairres grouped together, not all of them would have +4 armor. Archers could still pick off stragglers if used correctly.
I feel like the Roman units need some balancing and revamps, and I think this is a good start to make them more useful, and of course fun to use and fight.

How do others feel about this idea?