I think target audience matters a lot. When I think of Hero Arena, I think of the days in my childhood 12 to 15 years ago when my family would get together for the holidays and somebody would make a hero representing each of us and put them in a giant terrain box, and then we would attack a hero with a peasant to select it.
Then, along with the secret heroes representing each of us IRL people as our avatar, there would be a bunch of random mook heroes who were totally pointless and inferior. They were normally there as a distraction, and allowed everyone to pretend to play a "balanced" game.
But obviously the real way to play was always to pick the hero that represented you. We all shared maps around, and so even before I touched the map in the editor somebdy already put the original Retera hero included in the map with the Retera custom model from my maps, because anybody from our group working on the arena would just know that was the model for any Retera hero. The winner of every match was almost always whoever edited the map last and put in the most recent hero, since that would be the most powerful (since it was the most recent, and was before it was forced to be nerfed).
So, an obvious part of what I am describing is that it was totally not balanced. Beyond a certain point, balance was not necessary, and it would have been a grueling waste of human time to try to create the balance. The point of the Hero Arena is to make an entertaining combat using the existing systems of Warcraft III without having to invent a new system. But in the end, the existing systems of Warcraft III are actually not created to do a balanced hero battle but rather to do RTS gameplay, so if you are making quickly made fun maps like we all did back then it means your design will converge on the Blademaster/Demon Hunter/Warden- based melee Agility characters beating out all of the others typically.
If you think about it, it kind of makes sense. Without a grotesque amount of effort to change the system, intelligence adds mana (which can lead to better ability usage and eventually damage sometimes) and strength adds to health (which can add survivability) but agility increases attack speed (which can lead to better damage) and also armor (which can add survivability) so if you increase hero level without bound, the agility heroes eventually reach a power tier of their own where all others are left behind. Plus, if you add life stealing attacks, it can help cover for when they have less hit points than strength heroes.
So unless you're making some tactics arena where everyone has identical heroes and dodge each other's skill shots or some other polished, complex alien system, the default WC3 system won't be balanced and in my opinion it's better to just have fun.
But again, in order to do that, if you're going to laugh at how broken your creation is then what matters is who you're laughing with.