On the round robin, of course you can modify it however you want, but it still has to maintain some verisimilitude. For example, the last game I ran, the first DM had us in a tavern where we were listening intently to this wonderful storyteller weave interesting stories, and this guy comes in, and starts talking crap to the storyteller, telling him that he can't run forever, and that the he's finishing things once and for all, yadda yadda. This guy also has some "thugs" that accompany him. The storyteller vanishes, and the bad guy leaves, only to have the thugs do the dirty work.
So then I take over. The "thugs" start hitting the PCs really hard (I'm rolling d6s for fists) and one of the players is like "that's not right..." and then this illusion breaks down, revealing the thugs to be zombies. Oh, the look on the previous GMs face was priceless! Since he didn't intend for them to be zombies, but there wasn't anything stopping me from turning them into zombies! So, you work with what you have, and things change as you go. You can't just take over the reigns and say "suddenly you find yourself on a ship!" No, it doesn't work like that.
I use d20 rules for round robin just out of convenience. It's not tied to that. I could use HARP rules, but fewer people are familiar with those, and you have to be GM to play in the round robin game.