Current way of increasing difficulty is plain and simple - by increasing the attack damage of Dire creeps, and reducing attack damage of Radiant heroes and creeps.
Hereby, I suggest to brainstorm our way out of that.

About heroes:
- Do not alter hero damage at all, focus on enemy creeps instead.

About creeps:
- Increase the creep amount with each next difficulty INSTEAD of the damage bonus. And, to prevent the fact, that people would just use more cleave to deal with that - rebalance the existing creeps to be more damaging\beefy, have random attack range(for ranged creeps), and random attack speed and movespeed(for melees).
- Give ranged creeps a special aura, that would remove the buff\debuff movespeed from the creeps they're at the same team with. That would prevent creeps from scattering along the line.
- Bosses might have a cleave\radius of attack, just like Dragon Knight's splash, for example. The radius might have slight increases higher the difficulty goes.
- Boss creeps might have a "Wasp active ability" they only activate when near enemy heroes, allowing them to pass through buildings\walk up cliffs(skill might only have duration of 3, with cd of 8, with Winter Wyvern's tree breaking effect at the end of the duration). Their size clearly allows them that kind of behaviour.
- Slower creeps might have their spawn half-way closer to the Radiant base. Zombies, for example, might have their own spawnpoints in the Radiant jungle.

Suggest your ideas.