1. yes, items and potions would be a good addition, as karghool said. however, you prolly dont want the items and pots to be overpowering, just a lil added bonus.
2. You could also maybe add different type of shamans (fire shaman, ice shaman, wind shaman, etc) and you can switch the one you have out for a diff type at any time (and you wont lose your level or items or anything like that). Balancing will be critical when you do that as well.
3. you could also have mini-duels every 5 or 10 min, and a dueler is RANDOMLY chosen to fight. He can either fight another shaman or a creep, and which ever shaman wins, or if he beats the creep, maybe gets a little gold or exp boost, or even that shaman's team gets a temporary armor or dmg boost.
4. a small off-shoot on the mini-duels would be that at the beginnig of the game, 2 shamans are randomly chosen and put together to fight. the winner goes back into the game, the loser stays in the arena. after 45sec-1min, a new shaman fights the loser. if the loser wins, he goes back into the game. if the loser loses again, he has to wait another 30 sec before going back into the game, and 15 sec later, 2 new shamans are chosen and the process will repeat itself
There are some ideas for you to ponder