A lot of the ideas are not quite what we're looking for, but that's okay. Just give us a ton and we'll sort out which ones we like.
I'd still like to see object limit in it, but the fact you like time limit is fine with me.
And what about game mode customization? Where the player can customize a game mode?
Just throwin' some out.
The object limit could probably be a factor in several game modes, rather than a single mode in itself. The Mob mode might work well with an object limit, where you have to build a machine that can kill a huge amount of ragdolls at the same time, but you can only use so many pieces. That would probably work better than a time limit, for Mob mode specifically.
Game mode customization is an interesting idea, but I'm not sure how it would work. I need to know the scope of what kinds of game modes are possible in order to code a game mode editor. If you could describe some of the different types of custom game modes the player might come up with, that would help.
Game mode!
Obstacle Mode - You build a contraption as normal but once youve killed him once he moves to a different place. After the third kill he might be protected by a Short wall ( Short enough to throw a cannonball in ) And you have to build around it?
So your contraption would stay there, but the ragdoll would be somewhere else, and you'd have to add to it or rebuild? The idea of the ragdolls having some sort of defense is interesting. It could motivate solutions like using a cannon to destroy defending structures. Interesting idea.