Imlement "hold piece" functionality as a game setting. This allows for more strategic gameplay by queuing one piece.

Special to mess with target players inventory. Exactly how needs to be decided.

Randomize player positions when starting a new game.

Option to play with hidden identities.

Add sudden death settings when creating room.

http://tetris.wikia.com/wiki/T-Spin_Guide

Implement "Random generator" according to http://tetris.wikia.com/wiki/Random_Generator

Add inventory capacity as a game setting.

Implement custom key repeating, instead of relying on the systems key repeat.

Music

Perhaps different music depending on field height or current level.

Sound effects

Start game / game over
Moving / rotating block?
Removing lines
Specials

Show a configurable amount of "next pieces".

Require 50% or more to start game instead of just one guy clicking start?

To prevent a game from being stuck in waiting for players, maybe we should kick players out of the room when they have been idle for more than 1 minute.

Some variants implement a small delay when clearing lines. The delay is same regardless of how many lines are cleared, this will reward players going for tetris by giving them less delay overall.

Entry delay is the time between a piece has been locked and a new piece starts falling from the top. Make entry delay a game setting, so that it can be set per room.

Ability to have team enabled rooms. Friendly fire addlines are blocked and players cooperate to win over the other team(s).

Crazy idea: Limbo mode when a player dies. Players in limbo can be revived by teammates with revive special.