One of the things that I dislike on servers is that people spawn in, then stay close to their original spawn point, never exploring the rest of the world.
Because of this I have wished for the ability to have people spawn in at random locations on the already explored map OR within a predetermined area of a certain size (for example, a 1000x1000 area with 500,500 as the center). This would encourage exploration on servers that are intended for that, but it would also allow for people to start building where they are on a creative server, or to just set up a home. Ideally, the mod would avoid having people spawn in water or other dangerous places like 100m in the air or in lava. :-)
Of course, you can't have people spawn on a random place each time they log in, so only their very first spawn would be random, all subsequent spawns would be where they were last (which is already normal behavior).
Optionally an object could be given (like a Travelnet node or so) that when placed will provide the user with a list of destinations although that would be something for a different mod IMO.
Just a few thoughts that popped up in my head, but I would definitely use a mod like that. Even if it would only spawn new players at a random spot.
I have no Lua skills whatsoever, so I hope that this idea will inspire someone to create a thing like this.