You could add a lot of replayability value by adding upgrades for coins (total seconds survived) and then upgrading +1% skill for e.g. double bullet, enemy missfiring, homing bullet, survive on hit etc.
Destroyed other civilizations and then the game continues indefinitely. Also, Bachus has Favor multiplier instead of addition, so it bring Favor to a very deep negative value when there is unhappiness (like -10k and more).
You should enable the player to attack when all units die. Depending on the player's current rank, that creature/unit enters the battle as the last resort.
Combining items to become stronger would be a great idea
e.g. 10 Hitstreak skillbooks create Hitstreak LVL2 with 30% chance to hit again, 10 Bronze Armors give one LVL2 Bronze armor with 30% life boost etc.
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