For the winner's icon reward...
How about a "forum icon" size version of the Shard Golem in-game card art?
That could work if someone wants to make it.
---
Also, clarifying 'attacking':
When sending in your actions you'll state who you are attacking. If you are adjacent (or on the same square) to them at the beginning of the turn you attack them during the current turn. If you end the turn adjacent or in the same square as them, you'll attack them the following turn.
If two players attack each other, they are both counted as 'attackers'.
If two players both attack a 3rd player, the closer player will get priority.. and the other player will attack on the following turn. If they're the same distance away, they will fight each other first, then the 3rd player on the following day.
If a player with all shards is forced to battle on consecutive turns, they will move one square closer to the middle for each consecutive battle won.