Considering implementing it in Kotlin on Android
Some kind of peer-to-peer matchmaking, probably using a QR code that you scan on one phone to find the other phone. Most likely the QR code is a URL to a central server that just gives you the IP of the phone you want to pair with, if that works.
Your phone remembers the URL for known opponents and re-looks it up when you want to play again.
Maybe some kind of notification if a friend challenges you.
A fighting game with one button. The longer you hold, the harder the hit, but while you are holding, you are vulnerable.
Themes e.g. "banker vs anarchist"
Most likely each player is holding a big bat and health s measured e.g. by size of bump on head, or amount of gore.