We could do it like many other Survival games for warcraft 3. We have a shop at the start of each player. You start with a dummy unit, and at the shop you get to buy a hero, and troops. You could buy the most expensive hero, or a cheaper one + troops. Maybe even promotions.
Once you buy a hero it replaces the dummy unit, and you can start killing enemies. (No cities just barbarians and scenario based enemies).
The scenario could create waves of enemies. And you could get gold for every enemy defeated and you could buy more troops, promotions etc.