I think the only things that fans should design are the bosses.
...But, since we are on topic, I haven't played Legends 2, but judging from the art style of Legends and seeing similar symbols and structures in MM Zero, I can assume that Legends is centuries into the future of the other games. I think it'd be neat to stumble acros actual ruins of the old civilizations. Not the dark corridors and dungeons that the reaverbots are in, but actual remnants of a city long past, with various ancient mechaniloids sitting still after centuries of disrepair. Perhaps rusted mets and Sniper Joe's still lumber around, creaking along and following orders given by people long since gone. If there were ever an easter egg in Legends, that'd be it for me.