Developing the server side logic for sending the data to the the client side efficiently.
Time constraints..Wanted to add authorization for frequent players, audio on/off etc.
Adding mobile view for the website. It has been implemented but I recommend playing it on a bigger display such as laptop, etc for a better user experience.
Not a challenge but the video which has captions to explain has low quality(Link given in the video demo description).
The video with better quality without captions Link:

The game website is deployed at https://pokemonio.netlify.app/ have fun playing it with your pokémon friends!