If 1999 Mode wasn't challenging enough for you, the developers have a little somethin' for ya.
Irrational Games included the much more difficult 1999 Mode in Episode One of the Bioshock Infinite downloadable content, Burial at Sea. People liked it so much that the designers have confirmed that the even more taxing "1998 Mode" will be available in Episode 2.
The second episode will be ready on March 25 for the PlayStation 3 and Xbox 360 and if you're up to it, go ahead and tackle that mode. Said creative director Ken Levine:
"In Burial at Sea–Episode Two we put a focus on balance and stealth mechanics. As we were developing this new style of gameplay, we started to see people self-impose non-lethal playthroughs. Given the fan reception of 1999 Mode, we thought it would be cool to give them another way to play Burial at Sea that challenged their mastery of stealth tools."
If you click the link above, you'll see a picture that – if you're old enough – should strike you as familiar. That should remind you of the PC box for the original Thief PC game, which launched back in 1998. Ironically, the Thief reboot just released this week.
Related Game(s): Bioshock Infinite