Your father wanted you to have it...
OBI's line "... more powerful than you can possibly imagine..." has been discussed recently, but I'm also intrigued by OBI's claim that Anakin wanted Luke to have his lightsabre.
Up front, this could well have been no more than a lie to hook Luke into the idea, but I wonder if there is more here, too.
That scene has always been my favourite from the entire series. I love the way that droids, speeders, aliens, monsters and space ships are treated so casually and matter-of-factly prior to the sabres appearance so that when Luke asks what it is we know it's something VERY special, even in a galaxy where so many other over-worldly things are taken for granted. The jedi intrigue is fixed. Wonderful!
But what if Anakain did leave the sabre for OBI to pass on? Would that mean Anakin abandoned being a jedi prior to his fall, perhaps to seek refuge himself. Perhaps he isn't pushed into the lava at all - would it make more sense if he jumped in an attempt to avert his destiny? How dramatic a scene would that be - OBI and Anakin battling, Anakin knowing he has little influence over things, knowing his foe is his friend, throws himself into the pit in a lastditch attempt to save himself.
And then Palpy claws him out, fixes him up and enslaves him. A far more sympathetic Vader, if that's concievable...