Extend the arena game by showing more information about all the characters in the main GET /game page.
The arena game loses all its state when the program is restarted. A typical technique for a web application is to store all the state in a relational database. Think of other ways. What would be the simplest way? Implement it.
Implement a pause in the area game so that people cannot just spam the attack link.
Make the game prettier: serve a static CSS and link it to the templates.