I've only ever played OC, though I have played all the way from totally free-form to Champions. The most fun I had was with ending up at Xavier's - because of tie-ins, I suppose. An OC-only game will need a theme setting that probably requires a bit more work?
Grid - People still use one? I see a bunch of residences. When people want a scene they usually set it up on a channel or via pages or even +rp and off they go. Even in FC games, players tend to be separated by power levels, find and tag into plots they prefer - not wander into Galactus vs the Fantastic Four scene on the grid somewhere.
System. I find most systems stifle creativity, not invite it. Also, systems always invite min/max or people looking for the loophole. That being said, I've always been more attracted to a story, rather than a system. I say more, not only. Main hurdle to any MU* using a system not already boxed is making it - and that's just the main hurdle.
Without going into the how or why, the last super powered campaign I was involved with used a simple, free system that ended up being deceptively deep. We used Risus (not 2.0, but 1.5), with many of the optional rules - in particular Funky Dice. The basic premise of Risus is using @cliches. A cliche can be anything. They rank from 1 to 6, etc, etc. I suppose the main point of this paragraph is the players really enjoyed the total flexibility of concept. It ended up much like a Wild Talents game with Lite rules.
Not sure such a close-knit group of players translate over to a MU, though. Nobody in our group was like, @I Win Every Time, or @Can't Touch This. (Nor would it matter in a Risus game) But sometimes I do wonder if it doesn't really matter. People find their joy and avoid things they don't like.
I find as much joy in bringing somebody's idea to life as my own. When the two collide, it's a blast. Make it, not sure about 'they', but I will come - or at least peek.