Don´t know if I am the only sceptic, but I´m pretty sure if you link 2 games, balance goes out of the window.
1. If you are good in the campaign mode, it will make for dull battles.
2. If you are good at winning difficult battles, it will unbalance the campaign.
This is a problem to a lot of designs, but Pike and Shot Campaigns had a good way (and historical) way around it. I've also tried to implement a similar system in Tides of Conquest.
If the armies are not relatively evenly matched, no decisive battle takes place. The AI player simply marches off and does not fight, a la Fabius, the Germans in times of Roman dominance, or de Guesclin. The player then faces the time-consuming process of subduing the enemy territory through guerrilla warfare and sieges. This sucks for both sides economically, but at least offers the weaker side a good chance of survival.
Also, if you march too big an army into enemy territory, or one with too many horses, hunger will take care of it. Any area the size of, say, Burgundy or Normandy or Syria or Cisalpine Gaul should have at least 500,000 inhabitants and, on defense, could probably field at least 30,000 troops. That would be large enough to oppose on relatively equal terms any army that can feed itself in their territory.
It was HARD and expensive in the ancient world to conquer territory. Raiding was easy, staying took resources. Good campaign play should offer the player the CHANCE to conquer a territory. You still need to seal the deal with a battle and/or a hard-fought siege or guerrilla campaign.
A lot of ancients games fumble asymmetrical confrontation between large and small powers, but it ought be both fun and historical. Battles (or campaigns of subjugation, anyway) are almost never won by the side that brings the most resources. Resources allow you to sustain an army in enemy territory long enough to have the chance to subdue it. You still must succeed tactically as well.
Ideally, for conquest, tactics and strategy should be two separate hoops to jump through, and the one can't be used to compensate for the other.
I hope Empires takes a similar approach to P&S Campaigns in this regard.