I often think about this one: What if only YOU exist and all other people you see/know/hang out with, etc. are really only a part of a "plan" some higher being had in mind to "test" you in this life. So other people are not really experiencing life the way you do...they are part of YOUR plan...and are not "real" like you are.
Well, what do you expect - starting a "What if" thread?!?! LOL