Change white background color to gray. Show current time in the Leaderboard. Countdown sound in Battle Royale and new Teams mode (can be disabled editing the script).
Tested in Chrome with Tampermonkey.
Sound seems to only work when the agar.io tab is the active tab and Chrome is not minimized. You can circumvent this limitation by using a dedicated browser window only for agar.io and keeping it in the background (not minimized), while using another one for everything else. The countdown sound will alert you when the game is about to start.