Mix AOS and ORPG.
Start off with the standard computer-controlled allied creeps/towers that are weak. Players can purchase a builder unit to make really good creeps/towers (maybe a little bit like PRESCHOOL WARS)
An AOS that is about player skill and not item cheesing
More than 2 teams
Ability to break alliances and even try to win the whole game solo, or backstab your team
Phase out the economy mini-game. "Denying" is stupid and breaks the whole concept of an epic battle. Maybe have alternate ways of making gold, such as controlling the map rather than farming kills.
An AOS where the power of creeps doesn't become so trivial late game. Entire armys of multiple creep waves shouldn't fall to one hero with 3 battlefuries who probably spent all game killing kobolds in the woods.
The final objective is to kill a BOSS UNIT rather than a building. Like Kil'jaedan or Mannoroth or something, not a ziggurat with a creepy name.
Have a morale system. For example, if all the heroes on one team are dead, then give that side's creeps a "leaderless - low morale" debuff. Have army-wide morale buffs based on the progress of the "war" like "last stand" when defending the main objective or "FOR NARNIA, AND FOR ASLAAAAN" if you just killed a tower.
Be very careful about cheesing attributes, such as movespeed easily getting stacked to 522 and attack speed stacking to 500%. Acquiring very high levels of one thing should severely gimp others (or just be impossible). Many (all?) AOS maps reward players for just pumping one stat; I think this is dumb.
Examples of this:
DOTA: very easy to get ridiculous attack speed, without gimping anything else.
HOSK: Regen aura + endurance aura + a dozen kinds of boots that all stack means obtaining maximum movespeed is trivial at best.