In a game called Legion they had an interesting idea where if an army was spending a large amount of time in enemy territory, they begin to suffer (minimal) losses. This makes sense since because every army has men who are sick or injured or killed by locals when looking for food. The deeper you go into enemy land, the more casualties you have. The rate of Death is accelerated in winter, when food is scarce. This might be an interesting twist in RTW. Say you're playing Carthage and you want to recreate Hannibal's feat of crossing the Alps and attacking Rome. In the present version, going through the pass that leads to Mediolanum does not have you suffer any losses, even in winter, as does crossing the dessert in the height of Summer.