[Taqwus]

I've been idly wondering whether it'd be possible to model troops falling back without actually routing first.

To take an example from another computer game, the "Total War" engine -- at least, the incarnation that features in the "Shogun:TW" game -- permits the outcome of 'push' from two soldiers fighting each other: one soldier forces the other a bit backwards, but the fight continues on both parties.

For a more historical perspective, if we take Machiavelli's "Art of War" as an authority on Roman tactics when he quotes his contemporary Fabrizio (an assertion which I'm not qualified to properly assess, not having studied this in any detail), the different lines in formation might fall back and join with lines previously held in reserve during battle, and continue the fight from there.

These are most likely two different situations -- falling back as an individual, and falling back while trying to preserve some semblance of organization as a line. Adding either would add a bit of flavor over the currently binary advance-or-flee model. I'd also agree that having light infantry -- velites! -- fire and fall back (either through gaps or along the flanks) would add a reasonable option and go a long way towards making them something other than patrollers and garrison troops.