My response, lightly edited:
Are you willing to make the "massive" be satisfied by 109 players?
If so:
- 29 players can play on a single full sized keyboard if they're all given three keys each (left, right, forward) and you exclude keys like ctrl, alt, shift, etc. since those can issue system commands
- an additional player will be able to use the buttons on any standard three button mouse
- an additional 80 players can use gamepads since that's about the theoretical limit you'll hit. Those gamepads will be exceptionally crowded
- login details will require a unique ID based on a string of those three available inputs, both for usernames and passwords, but passwords could just as simply be something like "forward forward forward forward forward forward forward" or something. Think about usernames as if they're being entered on a sideways high score name wheel in an arcade game
- the map would scale outwards based on the bounds of the player positions
- as for the gameplay itself, just scatter random weapons around the map and, on pickup, they just start firing in direction the player is facing. Technically this is more of a battle royale than an MMO, though you can probably make the case that a battle royale is a subset of an MMO
- when only one person is standing, restart the round. Maybe make everyone have to log back in if you want to be a real motherfucker
- you might be able to get up to 110 players if you also use mouse direction, which would also take away the mouse player's advantage
sure, but that's not nearly as funny as crowding 5 people to every gamepad, 2 people to a mouse, and 29 people to a single keyboard
