Some thoughts:
Experience System
As much as I like the idea, a problem with this is that camping, instead of combat, is rewarded. The reason why most hero arenas simply "work" is because players are rewarded by being active. I suggest you use a combination of the two systems.
Hero Stats Balance
Heroes having the same stats is, of course, perfectly "balanced". However, it is also boring. Bear in mind that if heroes have different abilities, those are innately unbalanced. A game will never be truly balanced, that is why many perceived balanced games follow a "rock-paper-scissors" balance philosophy.
As two excellent recent examples; both Diablo 3 and Guild Wars 2 follow the idea that everything should be similarly balanced. Any class in GW2 can tank/DPS, classes exist more as aesthetic preference. Any class in Diablo 3 is equally powerful against swarms/ranged/giant/etc creeps. And while this is "balanced", it is also more boring that other games. Many enjoy it of course, but I strongly suggest you don't limit yourself to this balance philosophy.