1. Distance to capital. As your empire get larger and larger, the higher the unrest is in relation to your capital. Research, Magic, Garrison, and Infrastructure should help with this.
2. Unrest causes revolts in towns
3. The larger your stack, the slower your movement speed.
4. Allow multiple stacks to engage one stack. Take Mount and Blade's reinforcement system(I'll explain below). This will with out a doubt assist with killing off the stack of doom(Faster stacks can reinforce against slower stacks, but not vise versa)
5. Add a very expensive strategic spell that summons demons/dragons to attack the strongest faction. If the demons capture a city, they then begin to spawn darklings, etc.
6. Severely nerf the god-like weapons and spells so that one hero cannot wipe out an army by himself. (At best, a Hero should be as strong as 2-3X the strongest unit currently buildable by the player)
7. My Split-city idea. This should make larger cities progressively harder to defend as they get larger, by dividing the cities into sections based on tile placements, and each section can be captured individually. See: http://forums.elementalgame.com/420896/page/6
8. Factions can recruit Mercenary[men]/slave[fallen] designs from each other. I.e. if the player has access to far better technology, the enemy AI could hire a limited number of some of the designs from the player to gain access to some of these equipment to even the odds a bit. The player can do the same. Mercs should cost significant amount of gold, and Slaves should cost a resource called [slaves], gained by killing non-monsters.
9. Give a large bonus to Back Attacks. This should prevent a few super heavily armored soldiers to take on hordes of incoming weaker soldiers if they manage to "Flank".
Reinforcement system explanation.
1. ONLY Faster Stacks can reinforce Adjacent stacks(to the defenders) against slower stacks in both offense and defense. This give some usefulness to cavalry, and smaller stacks, as to discourage stacks of doom.
2. Slower stacks cannot reinforce any stacks against a faster stack attack. This not only discourages bigger stacks, but also gives some advantage to a more maneuverable, lightly armored force.
3. Reinforcement works by, calculating ratio between Attacker and Defenders. If say there are 18 [A]ttackers and 9 [D]efenders, then the battle will have a maximum of 9 attackers, and 5 defenders, as well as starting with that many. Either side can call reinforcements during the battle from the pool that didn't make it to the initial fight until they reach the max.[9 attacker, 5 defenders].
This should allow the strategy of overwhelming numbers to actually work, also too would the strategy to funnel enemy stacks into a thinner pathway to minimize the effects of overwhelming numbers.
10, Don't switch the ZoC immediatelly. Make it so that, for a few turns depending on the infrastructure and level of the city, you're still in a hostile environment. The defender, then, has a chance to retake the city fighting with the domestic bonuses. Can also port (forgot the spell name), right there, since it's still their territory. Also the defender doesn't lose their resources in the ZoC, if any.