Here's the deal, there are LOTS of hero arenas out there, barely very, very few that are of good quality...there is still room to do the same 'ol same 'ol and get away with it, as long as you put a lot of work into it and make the quality higher then the norm. So if you cannot think of fresh ideas, just simple increase the quality of existing ones.
remember:
-Detailed terrain
-Balanced heroes
-Single and multiplayer support (that means putting in AI compatability)
-Good use of imported stuff
-No rouph edges
Focus on those first before exploring ideas. As long as you got those, you'll have a quality map. And quality maps do good.
-VGsatomi