Probably a little, yeah.
He knows he's going to get a chance to flesh some of this stuff out in a SBS. Not everything needs to be addressed in the manga. And part of him is probably curious what parts of the story the fans are actually curious about or are actually paying attention to.
Mind, some of it probably is just leaving little story seeds behind for himself, to kind of challenge himself and see what he can come up with. You never know when inspiration will hit. So, sometimes it's fun to leave yourself little "mystery boxes" that you can think about for later. Some of it might be stuff he came up with on the fly. Some of it might be stuff he's planned out. And some of it even he might not be sure about yet. But, maybe he thinks he'll eventually think of a interesting angle on things that he can use down the line.
I mean, Law was supposedly never supposed to be as important to the story as he became. Part of the job of being a manga writer is improvising. Sometimes it's just fun to leave behind little "prompts" that you can call back to later, after you figure something out. Oda does plan a lot out. But, a lot of the manga is also made up on his whims. So, it's a bit of a balancing act of leaving yourself enough material to be able to make something with for later.