Changing levels, changing max health/start health, carrots on/off, shields on/off, wall climbing on/off, restarting a level, turning triggers on/off, instagib on/off, spawnobjects, etc. Plus allows a lot of options that drastically change gameplay in multiplayer, and would also affect single player. Although many of the options would probably just end up being used as cheats (as if jjgod wasn't enough), some would allow for some interesting options, and I personally have been looking forward to plus being applied to single player for quite some time now.