The problem with Luffy vs Saturn is the flashback doesnt take away the very blatant fact that Luffy wont be the one to save Bonney. He is like a shriveled vegetable right now, so the only real options for saving her are Kuma and the Robot, temporarily.
Sure, if the arc develops and somehow Kizaru is handled by someone else, then Luffy vs Saturn can happen. The issue is: Kizaru is getting back up (he clearly isnt knocked out) and Zoro is still in the dome fighting Lucci.
This is why, to me, it is very likely the Robot in the least holds Saturn back, and its why I believe it exists. Saturn may very well be the arc antagonist, but so was Big Mom at WCI and she wasn't dealt with until later.