I agree with Satrhan about requiring champions for any attack being a problem. But I've wanted non-combat roles for champions since before the first beta, and the idea of using them as a form of city-spam control seems brilliant to me.

The idea of hero required to attack is a way to simulate campaign armies. If it was for me, all troops would stay in cities and only get out when it's time to invade. But due to Elemental's current structure where many units can just wander on the board, one thing that could be done to simulate campaing armies is ask for a leader to be there. It's also nice because the unit stack figure is always the hero.

One of the advantage is that all roaming units of all players cannot be attacked while moving on the map or between cities. So they are safe. You could move an undefended settler surrounded by ennemy units and it will not get attacked. A stack of unit with a hero also gives a message that there is a possible invasion coming. Bandits would also require heroes too to be able to attack.

It has also the advantage of reducing the number of weenie battles. All battles becomes important. To avoid meaning less battles, I would even say that an army with a hero could have a zone of control of 1 or 2 square radius to prevent any units from passing through which again reduce the number of weenie battles. ONly an army with a hero will be able to enter the zone of control and challenge the opposing hero.

As for city management, It reminds me that in Gemfire, if you control a city without a lord, your control the city at 50% of your sovereign's capacity. It could be possible to do something similar so that you could expand more even if you do not have the personnel but will gain little benefit from it.

What you could do is stack limits on tiles unless you have a hero to command them, or they are in a city.  Overstacks that lose their hero will be controlled by the AI, and will try to head towards the nearest hero or town.

I really like this idea, in that it works without changing up the core gameplay too drastically.  Generally speaking, I keep one Champion in each city anyway, to really take advantage of their bonuses.
This could also change up City assaults, whereby Champions become the kind of "focus" for an attack, rather than all-out victory.  Killing a Champion leaves the city without its ruler, and it descends into chaos.  Grants an extra level of strategy to the proceedings, rather than just 'Stacks of Doom' that basically owns all. 

what will happen with the army if your only hero dies in battle?

 

Heroes don't die, they get wounded. Personally, I would not make hero take part of the battle, but rather have them has commanding officer. But if it's easier to just place them in battle, you just make dead heroes actually wounded. Once wounded, They cannot initiate battle but they can still move units on the field. If a unit gets attacked, the hero does not appear in battle, and any commanding bonus the hero would give will be ignored.

If you do this, you need ways to auto-genning up champions though, like through battles, births, and quests.

 

Yes, we need more hero input. In R3K, you gain them by searching your cities, by recruiting them from other players or sometimes heroes comes at your door to serve.

What you could do is stack limits on tiles unless you have a hero to command them

 

Stacking limit is another interesting option. Some hero stats could even determine the army size he can command. In R3K3, officer's leadership value determines the maximum number of soldier they can command.