Any proposal can be geared towards top and bottom.
One of my oldest suggestions
([Suggestion] Baby Titan / Second TITAN CHEST for rare (3* only) hero)
Alliance war is not a real war. This is a game. There are infinite ways SGG implement a tiered reward system.
On way would be to have alliances set a preferred rarity level ( 1* to 5* ). If your matchmaking is based on 3* heroes, then you can only use 1* to 3* heroes in war.
Challenge events, and tournaments, show many players enjoy rarity restrictions.