Great idea!

I think this could have exploitation (as you mentioned) but within boundaries (as you mentioned) this could be very fun! Maybe you could edit what the mechs you battle have. This could cause maybe a "heat level" or a "pro level".

But I'm voting yes on this one

I vote yes i want to fight 12 buggies xD but maybe liran will vote no because he is SuperMechs Developer xD

Creating a sandbox for other's to create off of takes a bit of time, but at the same time, it allows you to sit back without having to create any creative content yourself and gives the players EXACTLY what they want to keep themselves entertained. Retain your player base attention and they keep playing/paying.

But what's the payoff for doing such a thing? Just a little extra entertainment, or is it entertainment where you can create a masterpiece and challenge other's to try to take it on! Then you run into the issue of trying to ensure such things aren't abused. It'd definitely be hard to implement, but if thought out well and thoroughly, it could be an amazing addition

Voted yes. Sandbox levels for a game are always awesome